On 7/8/20 6:04 PM, Nick Desaulniers wrote:
> From: Jakub Kicinski <kuba@xxxxxxxxxx>
>
> When ur_load_imm_any() is inlined into jeq_imm(), it's possible for the
> compiler to deduce a case where _val can only have the value of -1 at
> compile time. Specifically,
>
> /* struct bpf_insn: _s32 imm */
> u64 imm = insn->imm; /* sign extend */
> if (imm >> 32) { /* non-zero only if insn->imm is negative */
> /* inlined from ur_load_imm_any */
> u32 __imm = imm >> 32; /* therefore, always 0xffffffff */
> if (__builtin_constant_p(__imm) && __imm > 255)
> compiletime_assert_XXX()
>
> This can result in tripping a BUILD_BUG_ON() in __BF_FIELD_CHECK() that
> checks that a given value is representable in one byte (interpreted as
> unsigned).
Looking at the 12 other callers of FIELD_FIT(), it's hard to
tell but it appears most of them don't supply constant _val
to the macro. So maybe relaxing this check is no great loss.
I feel like I need to look much more deeply into this to call
it a review, so:
Acked-by: Alex Elder <elder@xxxxxxxxxx>
> FIELD_FIT() should return true or false at runtime for whether a value
> can fit for not. Don't break the build over a value that's too large for
> the mask. We'd prefer to keep the inlining and compiler optimizations
> though we know this case will always return false.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/kernel-hardening/CAK7LNASvb0UDJ0U5wkYYRzTAdnEs64HjXpEUL7d=V0CXiAXcNw@xxxxxxxxxxxxxx/
> Reported-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Debugged-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> Changes V1->V2:
> * None
>
> include/linux/bitfield.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 48ea093ff04c..4e035aca6f7e 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -77,7 +77,7 @@
> */
> #define FIELD_FIT(_mask, _val) \
> ({ \
> - __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_FIT: "); \
> + __BF_FIELD_CHECK(_mask, 0ULL, 0ULL, "FIELD_FIT: "); \
> !((((typeof(_mask))_val) << __bf_shf(_mask)) & ~(_mask)); \
> })
>
>
