find_bad_casts() reports a cast between two randomized structure pointer
types with inform(), which renders as a "note:". It has done so since
the plugin was originally added, while using error() freely for attribute
misuse, UAPI structs, and version mismatches.
Clang's implementation of the same check has always been stricter:
it rejects such a cast as a full error. There is no reason for the GCC
side to be effectively silent about the same problem.
Build tested ARCH=x86_64 with CONFIG_RANDSTRUCT_FULL=y and GCC 14.2.0:
allmodconfig clean, and defconfig clean under three different random
seeds. A deliberate bad cast is still reported, now as a warning, at the
correct line and column.
Link: https://patch.msgid.link/20260903232438.60394-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>
Commit f9e7a75648 ("container_of: remove local __mptr variable") dropped
the "void *__mptr" temporary from container_of(). The randstruct GCC
plugin's find_bad_casts pass recognized the casts container_of() generates
by that variable's name:
const_tree ssa_name_var = SSA_NAME_VAR(rhs1);
/* skip bogus type casts introduced by container_of */
if (ssa_name_var != NULL_TREE && DECL_NAME(ssa_name_var) &&
!strcmp(DECL_NAME_POINTER(ssa_name_var), "__mptr"))
continue;
With the variable gone the suppression never fires, so every
container_of() whose container type is randomized now emits a note:
include/linux/container_of.h:23:9: note: randstruct: casting between
randomized structure pointer types (ssa): 'struct ocfs2_triggers' and
'struct jbd2_buffer_trigger_type'
fs/ocfs2/journal.c:524:16: note: in expansion of macro 'container_of'
The pass runs on GIMPLE, after folding, and for a member at offset 0 the
whole expression collapses to a bare copy that is indistinguishable from
an unsafe cast:
to_ocfs2_trigger (struct jbd2_buffer_trigger_type * triggers)
{
_2 = triggers_1(D); /* void * cast and subtraction gone */
return _2;
}
Match the type relationship instead. A cast from A * to B * is a
container_of() if B reaches a field of type A at offset 0 through a chain
of by-value members. The chain matters: container_of()'s member argument
may be a dotted path, as in ceph_inode(), which is container_of(inode,
struct ceph_inode_info, netfs.inode) and needs two levels. The search
is depth-bounded to 4 just in case, since real paths are generally one
or two members deep.
Requiring the cast happens at offset 0 is done because any other member
offset the subtraction survives folding and the cast's rhs is still
void *, which the pass already skips a few lines above.
A cast between two randomized types with no containment relationship is
still reported. Verified with:
struct cred *f(struct file *f) { return (struct cred *)f; }
which is still flagged with the patch applied.
Clang's implementation is unaffected. It checks the cast as written, and
both the old and new macros cast from void *, which is always permitted;
a genuinely bad cast is rejected there as a hard error rather than a note.
Build tested ARCH=x86_64 defconfig with CONFIG_RANDSTRUCT_FULL=y and GCC
14.2.0: randstruct notes 52 before, 0 after.
Fixes: f9e7a75648 ("container_of: remove local __mptr variable")
Assisted-by: Claude:claude-opus-5[1m]
Link: https://patch.msgid.link/20260903232438.60394-1-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>
When building the randomized replacement tree of struct members, the
randstruct GCC plugin would insert, as the first member, a 0-sized void
member. This appears as though it was done to catch non-designated
("unnamed") static initializers, which wouldn't be stable since they
depend on the original struct layout order.
This was accomplished by having the side-effect of the "void member"
tripping an assert in GCC internals (count_type_elements) if the member
list ever needed to be counted (e.g. for figuring out the order of members
during a non-designated initialization), which would catch impossible type
(void) in the struct:
security/landlock/fs.c: In function ‘hook_file_ioctl_common’:
security/landlock/fs.c:1745:61: internal compiler error: in count_type_elements, at expr.cc:7075
1745 | .u.op = &(struct lsm_ioctlop_audit) {
| ^
static HOST_WIDE_INT
count_type_elements (const_tree type, bool for_ctor_p)
{
switch (TREE_CODE (type))
...
case VOID_TYPE:
default:
gcc_unreachable ();
}
}
However this is a redundant safety measure since randstruct uses the
__designated_initializer attribute both internally and within the
__randomized_layout attribute macro so that this would be enforced
by the compiler directly even when randstruct was not enabled (via
-Wdesignated-init).
A recent change in Landlock ended up tripping the same member counting
routine when using a full-struct copy initializer as part of an anonymous
initializer. This, however, is a false positive as the initializer is
copying between identical structs (and hence identical layouts). The
"path" member is "struct path", a randomized struct, and is being copied
to from another "struct path", the "f_path" member:
landlock_log_denial(landlock_cred(file->f_cred), &(struct landlock_request) {
.type = LANDLOCK_REQUEST_FS_ACCESS,
.audit = {
.type = LSM_AUDIT_DATA_IOCTL_OP,
.u.op = &(struct lsm_ioctlop_audit) {
.path = file->f_path,
.cmd = cmd,
},
},
...
As can be seen with the coming randstruct KUnit test, there appears to
be no behavioral problems with this kind of initialization when the void
member is removed from the randstruct GCC plugin, so remove it.
Reported-by: "Dr. David Alan Gilbert" <linux@treblig.org>
Closes: https://lore.kernel.org/lkml/Z_PRaKx7q70MKgCA@gallifrey/
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/lkml/20250407-kbuild-disable-gcc-plugins-v1-1-5d46ae583f5e@kernel.org/
Reported-by: WangYuli <wangyuli@uniontech.com>
Closes: https://lore.kernel.org/lkml/337D5D4887277B27+3c677db3-a8b9-47f0-93a4-7809355f1381@uniontech.com/
Fixes: 313dd1b629 ("gcc-plugins: Add the randstruct plugin")
Signed-off-by: Kees Cook <kees@kernel.org>
Update code comment to clarify that the only element whose layout is
not randomized is a proper C99 flexible-array member. This update is
complementary to commit 1ee60356c2 ("gcc-plugins: randstruct: Only
warn about true flexible arrays")
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZWJr2MWDjXLHE8ap@work
Fixes: 1ee60356c2 ("gcc-plugins: randstruct: Only warn about true flexible arrays")
Signed-off-by: Kees Cook <keescook@chromium.org>
Commit 61f60bac8c ("gcc-plugins: Change all version strings match
kernel") broke parallel builds.
Instead of adding the dependency between GCC plugins and utsrelease.h,
let's use KERNELVERSION, which does not require any build artifact.
Another reason why I want to avoid utsrelease.h is because it depends
on CONFIG_LOCALVERSION(_AUTO) and localversion* files.
(include/generated/utsrelease.h depends on include/config/kernel.release,
which is generated by scripts/setlocalversion)
I want to keep host tools independent of the kernel configuration.
There is no good reason to rebuild GCC plugins just because of
CONFIG_LOCALVERSION being changed.
We just want to associate the plugin versions with the kernel source
version. KERNELVERSION should be enough for our purpose.
Fixes: 61f60bac8c ("gcc-plugins: Change all version strings match kernel")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/linux-mm/202205230239.EZxeZ3Fv-lkp@intel.com
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220524135541.1453693-1-masahiroy@kernel.org
With all randstruct exceptions removed, remove all the exception
handling code. Any future warnings are likely to be shared between
this plugin and Clang randstruct, and will need to be addressed in a
more wholistic fashion.
Cc: Christoph Hellwig <hch@infradead.org>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
The randstruct GCC plugin gets upset when it sees struct path (which is
randomized) being assigned from a "void *" (which it cannot type-check).
There's no need for these casts, as the entire internal payload use is
following a normal struct layout. Convert the enum-based void * offset
dereferencing to the new big_key_payload struct. No meaningful machine
code changes result after this change, and source readability is improved.
Drop the randstruct exception now that there is no "confusing" cross-type
assignment.
Cc: David Howells <dhowells@redhat.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-hardening@vger.kernel.org
Cc: keyrings@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
It's not meaningful for the GCC plugins to track their versions separately
from the rest of the kernel. Switch all versions to the kernel version.
Fix mismatched indenting while we're at it.
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
The initialization of "security_hook_heads" is done by casting it to
another structure pointer type, and treating it as an array of "struct
hlist_head" objects. This requires an exception be made in "randstruct",
because otherwise it will emit an error, reducing the effectiveness of
the hardening technique.
Instead of using a cast, initialize the individual struct hlist_head
elements in security_hook_heads explicitly. This removes the need for
the cast and randstruct exception.
Signed-off-by: Bill Wendling <morbo@google.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220407175930.471870-1-morbo@google.com
This changes security_hook_heads to use hlist_heads instead of
the circular doubly-linked list heads. This should cut down
the size of the struct by about half.
In addition, it allows mutation of the hooks at the tail of the
callback list without having to modify the head. The longer-term
purpose of this is to enable making the heads read only.
Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Reviewed-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <james.morris@microsoft.com>
GCC 8 changed the order of some fields and is very picky about ordering
in static initializers, so instead just move to dynamic initializers,
and drop the redundant already-zero field assignments.
Suggested-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Kees Cook <keescook@chromium.org>
This enables the automatic structure selection logic in the randstruct
GCC plugin. The selection logic randomizes all structures that contain
only function pointers, unless marked with __no_randomize_layout.
Signed-off-by: Kees Cook <keescook@chromium.org>
The NIU ethernet driver intentionally stores a page struct pointer on
top of the "mapping" field. Whitelist this case:
drivers/net/ethernet/sun/niu.c: In function ‘niu_rx_pkt_ignore’:
drivers/net/ethernet/sun/niu.c:3402:10: note: found mismatched ssa struct pointer types: ‘struct page’ and ‘struct address_space’
*link = (struct page *) page->mapping;
~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
The big_key payload structure intentionally stores a struct path in
two void pointers to avoid header soup. Whitelist this case:
security/keys/big_key.c: In function ‘big_key_read’:
security/keys/big_key.c:293:16: note: found mismatched rhs struct pointer types: ‘struct path’ and ‘void *’
struct path *path = (struct path *)&key->payload.data[big_key_path];
^~~~
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
This is another false positive in bad cast detection:
net/unix/af_unix.c: In function ‘unix_skb_scm_eq’:
net/unix/af_unix.c:1621:31: note: found mismatched rhs struct pointer types: ‘struct unix_skb_parms’ and ‘char’
const struct unix_skb_parms *u = &UNIXCB(skb);
^
UNIXCB is:
#define UNIXCB(skb) (*(struct unix_skb_parms *)&((skb)->cb))
And ->cb is:
char cb[48] __aligned(8);
This is a rather crazy cast, but appears to be safe in the face of
randomization, so whitelist it in the plugin.
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
The LSM initialization routines walk security_hook_heads as an array
of struct list_head instead of via names to avoid a ton of needless
source. Whitelist this to avoid the false positive warning from the
plugin:
security/security.c: In function ‘security_init’:
security/security.c:59:20: note: found mismatched op0 struct pointer types: ‘struct list_head’ and ‘struct security_hook_heads’
struct list_head *list = (struct list_head *) &security_hook_heads;
^
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: James Morris <james.l.morris@oracle.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
This randstruct plugin is modified from Brad Spengler/PaX Team's code
in the last public patch of grsecurity/PaX based on my understanding
of the code. Changes or omissions from the original code are mine and
don't reflect the original grsecurity/PaX code.
The randstruct GCC plugin randomizes the layout of selected structures
at compile time, as a probabilistic defense against attacks that need to
know the layout of structures within the kernel. This is most useful for
"in-house" kernel builds where neither the randomization seed nor other
build artifacts are made available to an attacker. While less useful for
distribution kernels (where the randomization seed must be exposed for
third party kernel module builds), it still has some value there since now
all kernel builds would need to be tracked by an attacker.
In more performance sensitive scenarios, GCC_PLUGIN_RANDSTRUCT_PERFORMANCE
can be selected to make a best effort to restrict randomization to
cacheline-sized groups of elements, and will not randomize bitfields. This
comes at the cost of reduced randomization.
Two annotations are defined,__randomize_layout and __no_randomize_layout,
which respectively tell the plugin to either randomize or not to
randomize instances of the struct in question. Follow-on patches enable
the auto-detection logic for selecting structures for randomization
that contain only function pointers. It is disabled here to assist with
bisection.
Since any randomized structs must be initialized using designated
initializers, __randomize_layout includes the __designated_init annotation
even when the plugin is disabled so that all builds will require
the needed initialization. (With the plugin enabled, annotations for
automatically chosen structures are marked as well.)
The main differences between this implemenation and grsecurity are:
- disable automatic struct selection (to be enabled in follow-up patch)
- add designated_init attribute at runtime and for manual marking
- clarify debugging output to differentiate bad cast warnings
- add whitelisting infrastructure
- support gcc 7's DECL_ALIGN and DECL_MODE changes (Laura Abbott)
- raise minimum required GCC version to 4.7
Earlier versions of this patch series were ported by Michael Leibowitz.
Signed-off-by: Kees Cook <keescook@chromium.org>
Kees Cook