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Date: Mon, 10 Aug 2020 12:17:22 -0700 (PDT) From: David Miller <davem@...emloft.net> To: ndesaulniers@...gle.com Cc: samitolvanen@...gle.com, kuba@...nel.org, stable@...r.kernel.org, masahiroy@...nel.org, ast@...nel.org, daniel@...earbox.net, kafai@...com, songliubraving@...com, yhs@...com, andriin@...com, john.fastabend@...il.com, kpsingh@...omium.org, elder@...aro.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org Subject: Re: [PATCH v2 net] bitfield.h: don't compile-time validate _val in FIELD_FIT From: Nick Desaulniers <ndesaulniers@...gle.com> Date: Mon, 10 Aug 2020 11:21:11 -0700 > From: Jakub Kicinski <kuba@...nel.org> > > 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@...r.kernel.org > Fixes: 1697599ee301a ("bitfield.h: add FIELD_FIT() helper") > Link: https://lore.kernel.org/kernel-hardening/CAK7LNASvb0UDJ0U5wkYYRzTAdnEs64HjXpEUL7d=V0CXiAXcNw@mail.gmail.com/ > Reported-by: Masahiro Yamada <masahiroy@...nel.org> > Debugged-by: Sami Tolvanen <samitolvanen@...gle.com> > Signed-off-by: Jakub Kicinski <kuba@...nel.org> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com> > --- > Changes V1->V2: > * add Fixes tag. Applied, thank you.
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