This is a note to let you know that I've just added the patch titled
powerpc: Hide empty pt_regs at base of the stack
to the 6.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
powerpc-hide-empty-pt_regs-at-base-of-the-stack.patch
and it can be found in the queue-6.5 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 0864512a5e75323e7bc7a2cfc915c18e25993b69
Author: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Date: Thu Aug 24 16:42:10 2023 +1000
powerpc: Hide empty pt_regs at base of the stack
[ Upstream commit d45c4b48dafb5820e5cc267ff9a6d7784d13a43c ]
A thread started via eg. user_mode_thread() runs in the kernel to begin
with and then may later return to userspace. While it's running in the
kernel it has a pt_regs at the base of its kernel stack, but that
pt_regs is all zeroes.
If the thread oopses in that state, it leads to an ugly stack trace with
a big block of zero GPRs, as reported by Joel:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.5.0-rc7-00004-gf7757129e3de-dirty #3
Hardware name: IBM PowerNV (emulated by qemu) POWER9 0x4e1200 opal:v7.0 PowerNV
Call Trace:
[c0000000036afb00] [c0000000010dd058] dump_stack_lvl+0x6c/0x9c (unreliable)
[c0000000036afb30] [c00000000013c524] panic+0x178/0x424
[c0000000036afbd0] [c000000002005100] mount_root_generic+0x250/0x324
[c0000000036afca0] [c0000000020057d0] prepare_namespace+0x2d4/0x344
[c0000000036afd20] [c0000000020049c0] kernel_init_freeable+0x358/0x3ac
[c0000000036afdf0] [c0000000000111b0] kernel_init+0x30/0x1a0
[c0000000036afe50] [c00000000000debc] ret_from_kernel_user_thread+0x14/0x1c
--- interrupt: 0 at 0x0
NIP: 0000000000000000 LR: 0000000000000000 CTR: 0000000000000000
REGS: c0000000036afe80 TRAP: 0000 Not tainted (6.5.0-rc7-00004-gf7757129e3de-dirty)
MSR: 0000000000000000 <> CR: 00000000 XER: 00000000
CFAR: 0000000000000000 IRQMASK: 0
GPR00: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR04: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR08: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR12: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR24: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR28: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
NIP [0000000000000000] 0x0
LR [0000000000000000] 0x0
--- interrupt: 0
The all-zero pt_regs looks ugly and conveys no useful information, other
than its presence. So detect that case and just show the presence of the
frame by printing the interrupt marker, eg:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.5.0-rc3-00126-g18e9506562a0-dirty #301
Hardware name: IBM pSeries (emulated by qemu) POWER9 (raw) 0x4e1202 0xf000005 of:SLOF,HEAD hv:linux,kvm pSeries
Call Trace:
[c000000003aabb00] [c000000001143db8] dump_stack_lvl+0x6c/0x9c (unreliable)
[c000000003aabb30] [c00000000014c624] panic+0x178/0x424
[c000000003aabbd0] [c0000000020050fc] mount_root_generic+0x250/0x324
[c000000003aabca0] [c0000000020057cc] prepare_namespace+0x2d4/0x344
[c000000003aabd20] [c0000000020049bc] kernel_init_freeable+0x358/0x3ac
[c000000003aabdf0] [c0000000000111b0] kernel_init+0x30/0x1a0
[c000000003aabe50] [c00000000000debc] ret_from_kernel_user_thread+0x14/0x1c
--- interrupt: 0 at 0x0
To avoid ever suppressing a valid pt_regs make sure the pt_regs has a
zero MSR and TRAP value, and is located at the very base of the stack.
Fixes: 6895dfc04741 ("powerpc: copy_thread fill in interrupt frame marker and back chain")
Reported-by: Joel Stanley <joel@xxxxxxxxx>
Reported-by: Nicholas Piggin <npiggin@xxxxxxxxx>
Reviewed-by: Joel Stanley <joel@xxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://msgid.link/20230824064210.907266-1-mpe@xxxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index b68898ac07e19..392404688cec3 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -2258,6 +2258,22 @@ unsigned long __get_wchan(struct task_struct *p)
return ret;
}
+static bool empty_user_regs(struct pt_regs *regs, struct task_struct *tsk)
+{
+ unsigned long stack_page;
+
+ // A non-empty pt_regs should never have a zero MSR or TRAP value.
+ if (regs->msr || regs->trap)
+ return false;
+
+ // Check it sits at the very base of the stack
+ stack_page = (unsigned long)task_stack_page(tsk);
+ if ((unsigned long)(regs + 1) != stack_page + THREAD_SIZE)
+ return false;
+
+ return true;
+}
+
static int kstack_depth_to_print = CONFIG_PRINT_STACK_DEPTH;
void __no_sanitize_address show_stack(struct task_struct *tsk,
@@ -2322,9 +2338,13 @@ void __no_sanitize_address show_stack(struct task_struct *tsk,
lr = regs->link;
printk("%s--- interrupt: %lx at %pS\n",
loglvl, regs->trap, (void *)regs->nip);
- __show_regs(regs);
- printk("%s--- interrupt: %lx\n",
- loglvl, regs->trap);
+
+ // Detect the case of an empty pt_regs at the very base
+ // of the stack and suppress showing it in full.
+ if (!empty_user_regs(regs, tsk)) {
+ __show_regs(regs);
+ printk("%s--- interrupt: %lx\n", loglvl, regs->trap);
+ }
firstframe = 1;
}
