[PATCH v2 1/5] arm64: report EL1 UNDEFs better

[Anshuman Khandual]

On 9/13/22 15:47, Mark Rutland wrote:
> If an UNDEFINED exception is taken from EL1, and do_undefinstr() doesn't
> find any suitable undef_hook, it will call:
> 
> 	BUG_ON(!user_mode(regs))
> 
> ... and the kernel will report a failure witin do_undefinstr() rather
> than reporting the original context that the UNDEFINED exception was
> taken from. The pt_regs and ESR value reported within the BUG() handler
> will be from within do_undefinstr() and the code dump will be for the
> BRK in BUG_ON(), which isn't sufficient to debug the cause of the
> original exception.
> 
> This patch makes the reporting better by having do_undefinstr() call
> die() directly in this case to report the original context from which
> the UNDEFINED exception was taken.
> 
> Prior to this patch, an undefined instruction is reported as:
> 
> | kernel BUG at arch/arm64/kernel/traps.c:497!
> | Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
> | Modules linked in:
> | CPU: 0 PID: 0 Comm: swapper Not tainted 5.19.0-rc3-00127-geff044f1b04e-dirty #3
> | Hardware name: linux,dummy-virt (DT)
> | pstate: 000000c5 (nzcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> | pc : do_undefinstr+0x28c/0x2ac
> | lr : do_undefinstr+0x298/0x2ac
> | sp : ffff800009f63bc0
> | x29: ffff800009f63bc0 x28: ffff800009f73c00 x27: ffff800009644a70
> | x26: ffff8000096778a8 x25: 0000000000000040 x24: 0000000000000000
> | x23: 00000000800000c5 x22: ffff800009894060 x21: ffff800009f63d90
> | x20: 0000000000000000 x19: ffff800009f63c40 x18: 0000000000000006
> | x17: 0000000000403000 x16: 00000000bfbfd000 x15: ffff800009f63830
> | x14: ffffffffffffffff x13: 0000000000000000 x12: 0000000000000019
> | x11: 0101010101010101 x10: 0000000000161b98 x9 : 0000000000000000
> | x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
> | x5 : ffff800009f761d0 x4 : 0000000000000000 x3 : ffff80000a2b80f8
> | x2 : 0000000000000000 x1 : ffff800009f73c00 x0 : 00000000800000c5
> | Call trace:
> |  do_undefinstr+0x28c/0x2ac
> |  el1_undef+0x2c/0x4c
> |  el1h_64_sync_handler+0x84/0xd0
> |  el1h_64_sync+0x64/0x68
> |  setup_arch+0x550/0x598
> |  start_kernel+0x88/0x6ac
> |  __primary_switched+0xb8/0xc0
> | Code: 17ffff95 a9425bf5 17ffffb8 a9025bf5 (d4210000)
> 
> With this patch applied, an undefined instruction is reported as:
> 
> | Internal error: Oops - Undefined instruction: 0 [#1] PREEMPT SMP
> | Modules linked in:
> | CPU: 0 PID: 0 Comm: swapper Not tainted 5.19.0-rc3-00128-gf27cfcc80e52-dirty #5
> | Hardware name: linux,dummy-virt (DT)
> | pstate: 800000c5 (Nzcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> | pc : setup_arch+0x550/0x598
> | lr : setup_arch+0x50c/0x598
> | sp : ffff800009f63d90
> | x29: ffff800009f63d90 x28: 0000000081000200 x27: ffff800009644a70
> | x26: ffff8000096778c8 x25: 0000000000000040 x24: 0000000000000000
> | x23: 0000000000000100 x22: ffff800009f69a58 x21: ffff80000a2b80b8
> | x20: 0000000000000000 x19: 0000000000000000 x18: 0000000000000006
> | x17: 0000000000403000 x16: 00000000bfbfd000 x15: ffff800009f63830
> | x14: ffffffffffffffff x13: 0000000000000000 x12: 0000000000000019
> | x11: 0101010101010101 x10: 0000000000161b98 x9 : 0000000000000000
> | x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
> | x5 : 0000000000000008 x4 : 0000000000000010 x3 : 0000000000000000
> | x2 : 0000000000000000 x1 : 0000000000000000 x0 : 0000000000000000
> | Call trace:
> |  setup_arch+0x550/0x598
> |  start_kernel+0x88/0x6ac
> |  __primary_switched+0xb8/0xc0
> | Code: b4000080 90ffed80 912ac000 97db745f (00000000)
> 
> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Reviewed-by: Mark Brown <broonie at kernel.org>

Reviewed-by: Anshuman Khandual <anshuman.khandual at arm.com>

> Cc: Alexandru Elisei <alexandru.elisei at arm.com>
> Cc: Amit Daniel Kachhap <amit.kachhap at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: James Morse <james.morse at arm.com>
> Cc: Will Deacon <will at kernel.org>
> ---
>  arch/arm64/kernel/traps.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index b7fed33981f7b..eac4f7a831750 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -494,7 +494,9 @@ void do_undefinstr(struct pt_regs *regs)
>  	if (call_undef_hook(regs) == 0)
>  		return;
>  
> -	BUG_ON(!user_mode(regs));
> +	if (!user_mode(regs))
> +		die("Oops - Undefined instruction", regs, 0);
> +
>  	force_signal_inject(SIGILL, ILL_ILLOPC, regs->pc, 0);
>  }
>  NOKPROBE_SYMBOL(do_undefinstr);