linux-kernel - Re: [PATCH v2 net] bitfield.h: don't compile-time validate _val in FIELD

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Message-Id: <20200810.121722.482500427015958955.davem@davemloft.net>
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.