On 12/14/24 07:06, Kees Cook wrote:
> GCC performs value range tracking for variables as a way to provide better
> diagnostics. One place this is regularly seen is with warnings associated
> with bounds-checking, e.g. -Wstringop-overflow, -Wstringop-overread,
> -Warray-bounds, etc. In order to keep the signal-to-noise ratio high,
> warnings aren't emitted when a value range spans the entire value range
> representable by a given variable. For example:
>
> unsigned int len;
> char dst[8];
> ...
> memcpy(dst, src, len);
>
> If len's value is unknown, it has the full "unsigned int" range of [0,
> UINT_MAX], and bounds checks against memcpy() will be ignored. However,
> when a code path has been able to narrow the range:
>
> if (len > 16)
> return;
> memcpy(dst, src, len);
>
> Then a range will be updated for the execution path. Above, len is now
> [0, 16], so we might see a -Wstringop-overflow warning like:
>
> error: '__builtin_memcpy' writing between 9 and 16 bytes from to region
> of size 8 [-Werror=stringop-overflow]
>
> When building with CONFIG_FORTIFY_SOURCE, the run-time bounds checking
> can appear to narrow value ranges for lengths for memcpy(), depending on
> how the compile constructs the execution paths during optimization
> passes, due to the checks on the size. For example:
>
> if (p_size_field != SIZE_MAX &&
> p_size != p_size_field && p_size_field < size)
>
> As intentionally designed, these checks only affect the kernel warnings
> emitted at run-time and do not block the potentially overflowing memcpy(),
> so GCC thinks it needs to produce a warning about the resulting value
> range that might be reaching the memcpy().
>
> We have seen this manifest a few times now, with the most recent being
> with cpumasks:
>
> In function ‘bitmap_copy’,
> inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2,
> inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2:
> ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading
> between 257 and 536870904 bytes from a region of size 256
> [-Werror=stringop-overread]
> 114 | #define __underlying_memcpy __builtin_memcpy
> | ^
> ./include/linux/fortify-string.h:633:9: note: in expansion of macro
> ‘__underlying_memcpy’
> 633 | __underlying_##op(p, q, __fortify_size);
> \
> | ^~~~~~~~~~~~~
> ./include/linux/fortify-string.h:678:26: note: in expansion of macro
> ‘__fortify_memcpy_chk’
> 678 | #define memcpy(p, q, s) __fortify_memcpy_chk(p, q, s,
> \
> | ^~~~~~~~~~~~~~~~~~~~
> ./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’
> 259 | memcpy(dst, src, len);
> | ^~~~~~
> kernel/padata.c: In function ‘__padata_set_cpumasks’:
> kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256]
> 713 | cpumask_var_t pcpumask,
> | ~~~~~~~~~~~~~~^~~~~~~~
>
> This warning is _not_ emitted when CONFIG_FORTIFY_SOURCE is disabled,
> and with the recent -fdiagnostics-details we can confirm the origin of
> the warning is due to the FORTIFY range checking:
>
> ../include/linux/bitmap.h:259:17: note: in expansion of macro 'memcpy'
> 259 | memcpy(dst, src, len);
> | ^~~~~~
> '__padata_set_cpumasks': events 1-2
> ../include/linux/fortify-string.h:613:36:
> 612 | if (p_size_field != SIZE_MAX &&
> | ~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 613 | p_size != p_size_field && p_size_field < size)
> | ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
> | |
> | (1) when the condition is
> evaluated to false
> | (2) when the condition is
> evaluated to true
> '__padata_set_cpumasks': event 3
> 114 | #define __underlying_memcpy __builtin_memcpy
> | ^
> | |
> | (3) out of array bounds here
>
> Note that this warning started appearing since bitmap functions were
> recently marked __always_inline in commit ed8cd2b3bd9f ("bitmap: Switch
> from inline to __always_inline"), which allowed GCC to gain visibility
> into the variables as they passed through the FORTIFY implementation.
>
> In order to silence this false positive but keep deterministic
> compile-time warnings intact, hide the length variable from GCC with
> OPTIMIZE_HIDE_VAR() before calling the builtin memcpy.
>
> Additionally add a comment about why all the macro args have copies with
> const storage.
>
> Reported-by: "Thomas Weißschuh" <[email protected]>
> Closes:
> https://lore.kernel.org/all/[email protected]/
> Reported-by: Nilay Shroff <[email protected]>
> Closes:
> https://lore.kernel.org/all/[email protected]/
> Acked-by: Yury Norov <[email protected]>
> Signed-off-by: Kees Cook <[email protected]>
> ---
> Cc: Nathan Chancellor <[email protected]>
> Cc: "Qing Zhao" <[email protected]>
> Cc: [email protected]
>
> v2: Make sure the expression statement ends with a single statement
> v1: https://lore.kernel.org/all/[email protected]/
> ---
> include/linux/fortify-string.h | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index 0d99bf11d260..1eef0119671c 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -616,6 +616,12 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t
> size,
> return false;
> }
>
> +/*
> + * To work around what seems to be an optimizer bug, the macro arguments
> + * need to have const copies or the values end up changed by the time they
> + * reach fortify_warn_once(). See commit 6f7630b1b5bc ("fortify: Capture
> + * __bos() results in const temp vars") for more details.
> + */
> #define __fortify_memcpy_chk(p, q, size, p_size, q_size, \
> p_size_field, q_size_field, op) ({ \
> const size_t __fortify_size = (size_t)(size); \
> @@ -623,6 +629,8 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t
> size,
> const size_t __q_size = (q_size); \
> const size_t __p_size_field = (p_size_field); \
> const size_t __q_size_field = (q_size_field); \
> + /* Keep a mutable version of the size for the final copy. */ \
> + size_t __copy_size = __fortify_size; \
> fortify_warn_once(fortify_memcpy_chk(__fortify_size, __p_size, \
> __q_size, __p_size_field, \
> __q_size_field, FORTIFY_FUNC_ ##op), \
> @@ -630,7 +638,11 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t
> size,
> __fortify_size, \
> "field \"" #p "\" at " FILE_LINE, \
> __p_size_field); \
> - __underlying_##op(p, q, __fortify_size); \
> + /* Hide only the run-time size from value range tracking to */ \
> + /* silence compile-time false positive bounds warnings. */ \
> + if (!__builtin_constant_p(__fortify_size)) \
> + OPTIMIZER_HIDE_VAR(__copy_size); \
> + __underlying_##op(p, q, __copy_size); \
> })
>
> /*This patch works for me. I tested it on PowerPC and x86-64 using GCC 13.X,
CONFIG_FORTIFY_SOURCE=Y and CONFIG_NR_CPUS=2048. So,
Tested-By: [email protected]
