From: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Date: Thu, 6 Aug 2020 11:29:39 -0700
> 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).
>
> 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>
> Acked-by: Alex Elder <elder@xxxxxxxxxx>
Please resend with an appropriate Fixes: tag, thank you.
