ACK: [B][C][SRU][PATCH 1/1] kprobes/x86: Fix instruction patching corruption when copying more than one RIP-relative instruction

[Connor Kuehl]

On 5/7/19 1:50 AM, Po-Hsu Lin wrote:
> From: Masami Hiramatsu <mhiramat at kernel.org>
> 
> BugLink: https://bugs.launchpad.net/bugs/1826385
> 
> After copy_optimized_instructions() copies several instructions
> to the working buffer it tries to fix up the real RIP address, but it
> adjusts the RIP-relative instruction with an incorrect RIP address
> for the 2nd and subsequent instructions due to a bug in the logic.
> 
> This will break the kernel pretty badly (with likely outcomes such as
> a kernel freeze, a crash, or worse) because probed instructions can refer
> to the wrong data.
> 
> For example putting kprobes on cpumask_next() typically hits this bug.
> 
> cpumask_next() is normally like below if CONFIG_CPUMASK_OFFSTACK=y
> (in this case nr_cpumask_bits is an alias of nr_cpu_ids):
> 
>  <cpumask_next>:
> 	48 89 f0		mov    %rsi,%rax
> 	8b 35 7b fb e2 00	mov    0xe2fb7b(%rip),%esi # ffffffff82db9e64 <nr_cpu_ids>
> 	55			push   %rbp
> ...
> 
> If we put a kprobe on it and it gets jump-optimized, it gets
> patched by the kprobes code like this:
> 
>  <cpumask_next>:
> 	e9 95 7d 07 1e		jmpq   0xffffffffa000207a
> 	7b fb			jnp    0xffffffff81f8a2e2 <cpumask_next+2>
> 	e2 00			loop   0xffffffff81f8a2e9 <cpumask_next+9>
> 	55			push   %rbp
> 
> This shows that the first two MOV instructions were copied to a
> trampoline buffer at 0xffffffffa000207a.
> 
> Here is the disassembled result of the trampoline, skipping
> the optprobe template instructions:
> 
> 	# Dump of assembly code from 0xffffffffa000207a to 0xffffffffa00020ea:
> 
> 	54			push   %rsp
> 	...
> 	48 83 c4 08		add    $0x8,%rsp
> 	9d			popfq
> 	48 89 f0		mov    %rsi,%rax
> 	8b 35 82 7d db e2	mov    -0x1d24827e(%rip),%esi # 0xffffffff82db9e67 <nr_cpu_ids+3>
> 
> This dump shows that the second MOV accesses *(nr_cpu_ids+3) instead of
> the original *nr_cpu_ids. This leads to a kernel freeze because
> cpumask_next() always returns 0 and for_each_cpu() never ends.
> 
> Fix this by adding 'len' correctly to the real RIP address while
> copying.
> 
> [ mingo: Improved the changelog. ]
> 
> Reported-by: Michael Rodin <michael at rodin.online>
> Signed-off-by: Masami Hiramatsu <mhiramat at kernel.org>
> Reviewed-by: Steven Rostedt (VMware) <rostedt at goodmis.org>
> Cc: Arnaldo Carvalho de Melo <acme at kernel.org>
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Ravi Bangoria <ravi.bangoria at linux.ibm.com>
> Cc: Steven Rostedt <rostedt at goodmis.org>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: stable at vger.kernel.org # v4.15+
> Fixes: 63fef14fc98a ("kprobes/x86: Make insn buffer always ROX and use text_poke()")
> Link: http://lkml.kernel.org/r/153504457253.22602.1314289671019919596.stgit@devbox
> Signed-off-by: Ingo Molnar <mingo at kernel.org>
> (cherry picked from commit 43a1b0cb4cd6dbfd3cd9c10da663368394d299d8)
> Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
> ---

Acked-by: Connor Kuehl <connor.kuehl at canonical.com>

>  arch/x86/kernel/kprobes/opt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
> index 1467f96..72e1286 100644
> --- a/arch/x86/kernel/kprobes/opt.c
> +++ b/arch/x86/kernel/kprobes/opt.c
> @@ -189,7 +189,7 @@ static int copy_optimized_instructions(u8 *dest, u8 *src, u8 *real)
>  	int len = 0, ret;
>  
>  	while (len < RELATIVEJUMP_SIZE) {
> -		ret = __copy_instruction(dest + len, src + len, real, &insn);
> +		ret = __copy_instruction(dest + len, src + len, real + len, &insn);
>  		if (!ret || !can_boost(&insn, src + len))
>  			return -EINVAL;
>  		len += ret;
> 

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