Re: [PATCH] arm64: prohibit probing on arch_kunwind_consume_entry()

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Feb 29, 2024 at 11:16:20PM +0000, Puranjay Mohan wrote:
> Make arch_kunwind_consume_entry() as __always_inline otherwise the
> compiler might not inline it and allow attaching probes to it.
> 
> Without this, just probing arch_kunwind_consume_entry() via
> <tracefs>/kprobe_events will crash the kernel on arm64.
> 
> The crash can be reproduced using the following compiler and kernel
> combination:
> clang version 19.0.0git (https://github.com/llvm/llvm-project.git d68d29516102252f6bf6dc23fb22cef144ca1cb3)
> commit 87adedeba51a ("Merge tag 'net-6.8-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
> 
>  [root@localhost ~]# echo 'p arch_kunwind_consume_entry' > /sys/kernel/debug/tracing/kprobe_events
>  [root@localhost ~]# echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
> 
>  Modules linked in: aes_ce_blk aes_ce_cipher ghash_ce sha2_ce virtio_net sha256_arm64 sha1_ce arm_smccc_trng net_failover failover virtio_mmio uio_pdrv_genirq uio sch_fq_codel dm_mod dax configfs
>  CPU: 3 PID: 1405 Comm: bash Not tainted 6.8.0-rc6+ #14
>  Hardware name: linux,dummy-virt (DT)
>  pstate: 604003c5 (nZCv DAIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>  pc : kprobe_breakpoint_handler+0x17c/0x258
>  lr : kprobe_breakpoint_handler+0x17c/0x258
>  sp : ffff800085d6ab60
>  x29: ffff800085d6ab60 x28: ffff0000066f0040 x27: ffff0000066f0b20
>  x26: ffff800081fa7b0c x25: 0000000000000002 x24: ffff00000b29bd18
>  x23: ffff00007904c590 x22: ffff800081fa6590 x21: ffff800081fa6588
>  x20: ffff00000b29bd18 x19: ffff800085d6ac40 x18: 0000000000000079
>  x17: 0000000000000001 x16: ffffffffffffffff x15: 0000000000000004
>  x14: ffff80008277a940 x13: 0000000000000003 x12: 0000000000000003
>  x11: 00000000fffeffff x10: c0000000fffeffff x9 : aa95616fdf80cc00
>  x8 : aa95616fdf80cc00 x7 : 205d343137373231 x6 : ffff800080fb48ec
>  x5 : 0000000000000000 x4 : 0000000000000001 x3 : 0000000000000000
>  x2 : 0000000000000000 x1 : ffff800085d6a910 x0 : 0000000000000079
>  Call trace:
>  kprobes: Failed to recover from reentered kprobes.
>  kprobes: Dump kprobe:
>  .symbol_name = arch_kunwind_consume_entry, .offset = 0, .addr = arch_kunwind_consume_entry+0x0/0x40
>  ------------[ cut here ]------------
>  kernel BUG at arch/arm64/kernel/probes/kprobes.c:241!
>  kprobes: Failed to recover from reentered kprobes.
>  kprobes: Dump kprobe:
>  .symbol_name = arch_kunwind_consume_entry, .offset = 0, .addr = arch_kunwind_consume_entry+0x0/0x40
> 
> Fixes: 1aba06e7b2b49 ("arm64: stacktrace: factor out kunwind_stack_walk()")
> Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>

Thanks for this!

Whoops; I had meant to make this __always_inline (or noinstr), but I evidently
messed that up. I don't recall any problem with making this __always_inline,
and that's preferable here to allow the compiler to fold some of the
indirection.

From a scan of stacktrace.c I don't see anything else that needs similar
treatment; the other functions lacking __always_inline and noinstr are safe to
instrument as they aren't core to the unwinder, and won't recurse into
themselves in a problematic way.

Given all the above:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Catalin, Will, are you happy to queue this as a fix?

Mark.

> ---
>  arch/arm64/kernel/stacktrace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 7f88028a00c0..b2a60e0bcfd2 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -247,7 +247,7 @@ struct kunwind_consume_entry_data {
>  	void *cookie;
>  };
>  
> -static bool
> +static __always_inline bool
>  arch_kunwind_consume_entry(const struct kunwind_state *state, void *cookie)
>  {
>  	struct kunwind_consume_entry_data *data = cookie;
> 
> base-commit: 87adedeba51a822533649b143232418b9e26d08b
> -- 
> 2.40.1
>