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Date:   Sat, 08 Aug 2020 14:16:57 -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, elder@...aro.org, ast@...nel.org,
        daniel@...earbox.net, kafai@...com, songliubraving@...com,
        yhs@...com, andriin@...com, john.fastabend@...il.com,
        kpsingh@...omium.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net resend] bitfield.h: don't compile-time validate
 _val in FIELD_FIT

From: Nick Desaulniers <ndesaulniers@...gle.com>
Date: Thu,  6 Aug 2020 11:29:39 -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
> 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>
> Acked-by: Alex Elder <elder@...aro.org>

Please resend with an appropriate Fixes: tag, thank you.

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