Re: [PATCH] ARM: fix race in for_each_frame

[黄吕强 (Lvqiang Huang)]

Dear Mark,
Thanks a lot for the reply. 

As said in last reply, sv_pc can be a module text, then more check needed.

And beside crash at 1003, we may also get crash at 1001, the frame is invalid. (The last sv_pv is valid and sv_frame is invalid), then more check needed. 

And we often show_data around the general proposal registers when kernel crash. When they contain an address mapping for a hw register but can’t access because clock gated, it will crash again because do_bad() is involved. (continuous crash in arm and hang at die_lock in arm64)

So, why not check the __ex_table in do_bad() ?

> 在 2019年11月12日，21:31，Mark-PK Tsai <mark-pk.tsai@xxxxxxxxxxxx> 写道：
> 
> The sv_pc, which is saved in the stack, may be an invalid address
> if the target thread is running on another processor in the meantime.
> It will cause kernel crash at `ldr r2, [sv_pc, #-4]`.
> 
> Check if sv_pc is valid before use it like unwind_frame in
> arch/arm/kernel/unwind.c.
> 
> Signed-off-by: Mike-SL Lin <mike-sl.lin@xxxxxxxxxxxx>
> Signed-off-by: Mark-PK Tsai <mark-pk.tsai@xxxxxxxxxxxx>
> ---
> arch/arm/lib/backtrace.S | 5 +++++
> 1 file changed, 5 insertions(+)
> 
> diff --git a/arch/arm/lib/backtrace.S b/arch/arm/lib/backtrace.S
> index 582925238d65..84f06381bbfb 100644
> --- a/arch/arm/lib/backtrace.S
> +++ b/arch/arm/lib/backtrace.S
> @@ -64,6 +64,11 @@ for_each_frame:    tst    frame, mask        @ Check for address exceptions
>        sub    sv_pc, sv_pc, offset    @ Correct PC for prefetching
>        bic    sv_pc, sv_pc, mask    @ mask PC/LR for the mode
> 
> +        mov    r0, sv_pc
> +        bl    kernel_text_address    @ check if sv_pc is valid
> +        cmp    r0, #0            @ if sv_pc is not kernel text
> +        beq    1006f            @ address, abort backtrace
> +
> 1003:        ldr    r2, [sv_pc, #-4]    @ if stmfd sp!, {args} exists,
>        ldr    r3, .Ldsi+4        @ adjust saved 'pc' back one
>        teq    r3, r2, lsr #11        @ instruction
> -- 
> 2.18.0


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