Re: [PATCH bpf] arm, bpf: Fix offset overflow for BPF_MEM BPF_DW

[Daniel Borkmann <daniel@iogearbox.net>]

On 4/10/20 12:17 AM, Luke Nelson wrote:
> This patch fixes an incorrect check in how immediate memory offsets are
> computed for BPF_DW on arm.
> 
> For BPF_LDX/ST/STX + BPF_DW, the 32-bit arm JIT breaks down an 8-byte
> access into two separate 4-byte accesses using off+0 and off+4. If off
> fits in imm12, the JIT emits a ldr/str instruction with the immediate
> and avoids the use of a temporary register. While the current check off
> <= 0xfff ensures that the first immediate off+0 doesn't overflow imm12,
> it's not sufficient for the second immediate off+4, which may cause the
> second access of BPF_DW to read/write the wrong address.
> 
> This patch fixes the problem by changing the check to
> off <= 0xfff - 4 for BPF_DW, ensuring off+4 will never overflow.
> 
> A side effect of simplifying the check is that it now allows using
> negative immediate offsets in ldr/str. This means that small negative
> offsets can also avoid the use of a temporary register.
> 
> This patch introduces no new failures in test_verifier or test_bpf.c.
> 
> Fixes: c5eae692571d6 ("ARM: net: bpf: improve 64-bit store implementation")
> Fixes: ec19e02b343db ("ARM: net: bpf: fix LDX instructions")
> Co-developed-by: Xi Wang <xi.wang@gmail.com>
> Signed-off-by: Xi Wang <xi.wang@gmail.com>
> Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>

Applied, thanks!