Re: [PATCH] MIPS: eBPF: Fix icache flush end address

[Daniel Borkmann]

On 03/01/2019 11:58 PM, Paul Burton wrote:
> The MIPS eBPF JIT calls flush_icache_range() in order to ensure the
> icache observes the code that we just wrote. Unfortunately it gets the
> end address calculation wrong due to some bad pointer arithmetic.
> 
> The struct jit_ctx target field is of type pointer to u32, and as such
> adding one to it will increment the address being pointed to by 4 bytes.
> Therefore in order to find the address of the end of the code we simply
> need to add the number of 4 byte instructions emitted, but we mistakenly
> add the number of instructions multiplied by 4. This results in the call
> to flush_icache_range() operating on a memory region 4x larger than
> intended, which is always wasteful and can cause crashes if we overrun
> into an unmapped page.
> 
> Fix this by correcting the pointer arithmetic to remove the bogus
> multiplication, and use braces to remove the need for a set of brackets
> whilst also making it obvious that the target field is a pointer.
> 
> Signed-off-by: Paul Burton <paul.burton@xxxxxxxx>
> Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Martin KaFai Lau <kafai@xxxxxx>
> Cc: Song Liu <songliubraving@xxxxxx>
> Cc: Yonghong Song <yhs@xxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: bpf@xxxxxxxxxxxxxxx
> Cc: linux-mips@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx # v4.13+

Good catch, applied to bpf, thanks!