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

[Mark Rutland]

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>
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: Mark Brown <broonie at kernel.org>
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 b7fed33981f7..eac4f7a83175 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);
-- 
2.30.2