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

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

[Paul Burton]

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@mips.com>
Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: netdev@vger.kernel.org
Cc: bpf@vger.kernel.org
Cc: linux-mips@vger.kernel.org
Cc: stable@vger.kernel.org # v4.13+
---
 arch/mips/net/ebpf_jit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/net/ebpf_jit.c b/arch/mips/net/ebpf_jit.c
index 76e9bf88d3b9..0effd3cba9a7 100644
--- a/arch/mips/net/ebpf_jit.c
+++ b/arch/mips/net/ebpf_jit.c
@@ -1819,7 +1819,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 
 	/* Update the icache */
 	flush_icache_range((unsigned long)ctx.target,
-			   (unsigned long)(ctx.target + ctx.idx * sizeof(u32)));
+			   (unsigned long)&ctx.target[ctx.idx]);
 
 	if (bpf_jit_enable > 1)
 		/* Dump JIT code */
-- 
2.21.0

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@mips.com>
> Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Martin KaFai Lau <kafai@fb.com>
> Cc: Song Liu <songliubraving@fb.com>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: netdev@vger.kernel.org
> Cc: bpf@vger.kernel.org
> Cc: linux-mips@vger.kernel.org
> Cc: stable@vger.kernel.org # v4.13+

Good catch, applied to bpf, thanks!