Re: linux-next: Tree for May 16 (security/landlock/ruleset.c)

["Mickaël Salaün" <mic@digikod.net>]

On Mon, May 19, 2025 at 11:19:53AM -0700, Kees Cook wrote:
> On Mon, May 19, 2025 at 05:29:30PM +0200, Mickaël Salaün wrote:
> > On Fri, May 16, 2025 at 07:54:14PM -0700, Randy Dunlap wrote:
> > > 
> > > 
> > > On 5/16/25 3:24 AM, Stephen Rothwell wrote:
> > > > Hi all,
> > > > 
> > > > Changes since 20250515:
> > 
> > Thanks for the report.
> > 
> > It is the same warning as reported here:
> > https://lore.kernel.org/all/202501040747.S3LYfvYq-lkp@intel.com/
> > 
> > I don't know what the actual issue is though.
> > 
> > > 
> > > on i386:
> > > 
> > > In file included from ../arch/x86/include/asm/string.h:3,
> > >                  from ../include/linux/string.h:65,
> > >                  from ../include/linux/bitmap.h:13,
> > >                  from ../include/linux/cpumask.h:12,
> > >                  from ../include/linux/smp.h:13,
> > >                  from ../include/linux/lockdep.h:14,
> > >                  from ../security/landlock/ruleset.c:16:
> > > ../security/landlock/ruleset.c: In function 'create_rule':
> > > ../arch/x86/include/asm/string_32.h:150:25: warning: '__builtin_memcpy' accessing 4294967295 bytes at offsets 20 and 0 overlaps 6442450943 bytes at offset -2147483648 [-Wrestrict]
> > >   150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
> > >       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
> > > ../security/landlock/ruleset.c:137:9: note: in expansion of macro 'memcpy'
> > >   137 |         memcpy(new_rule->layers, layers,
> > >       |         ^~~~~~
> > > 
> > > 
> > > Full randconfig file is attached.
> 
> The trigger appears to be CONFIG_PROFILE_ALL_BRANCHES, and GCC getting
> tricked into thinking check_mul_overflow() returns true:
> 
> In file included from ../arch/x86/include/asm/string.h:3,
>                  from ../include/linux/string.h:65,
>                  from ../include/linux/bitmap.h:13,
>                  from ../include/linux/cpumask.h:12,
>                  from ../include/linux/smp.h:13,
>                  from ../include/linux/lockdep.h:14,
>                  from ../security/landlock/ruleset.c:16:
> ../security/landlock/ruleset.c: In function 'create_rule':
> ../arch/x86/include/asm/string_32.h:150:25: warning: '__builtin_memcpy' accessing 4294967295 bytes at offsets 0 and 0 overlaps 6442450943 bytes at offset -2147483648 [-Wrestrict]
>   150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
> ../security/landlock/ruleset.c:137:9: note: in expansion of macro 'memcpy'
>   137 |         memcpy(new_rule->layers, layers,
>       |         ^~~~~~
>   'create_rule': event 1
> ../include/linux/compiler.h:69:46:
>    68 |         (cond) ?                                        \
>       |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    69 |                 (__if_trace.miss_hit[1]++,1) :          \
>       |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
>       |                                              |
>       |                                              (1) when the condition is evaluated to true
>    70 |                 (__if_trace.miss_hit[0]++,0);           \
>       |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
> ../include/linux/compiler.h:57:69: note: in expansion of macro '__trace_if_value'
>    57 | #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond))
>       |                                                                     ^~~~~~~~~~~~~~~~
> ../include/linux/compiler.h:55:28: note: in expansion of macro '__trace_if_var'
>    55 | #define if(cond, ...) if ( __trace_if_var( !!(cond , ## __VA_ARGS__) ) )
>       |                            ^~~~~~~~~~~~~~
> ../include/linux/overflow.h:270:9: note: in expansion of macro 'if'
>   270 |         if (check_mul_overflow(factor1, factor2, &bytes))
>       |         ^~
>   'create_rule': event 2
> ../arch/x86/include/asm/string_32.h:150:25:
>   150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
>       |                         |
>       |                         (2) out of array bounds here
> ../security/landlock/ruleset.c:137:9: note: in expansion of macro 'memcpy'
>   137 |         memcpy(new_rule->layers, layers,
>       |         ^~~~~~
> make[1]: Leaving directory '/srv/code/gcc-bug'

That's interesting...

> 
> 
> I'll take a look at ways to make either the overflow macros or memcpy
> robust against this kind of weirdness...

Thanks!

> 
> -- 
> Kees Cook
>