Patch "powerpc: Hide empty pt_regs at base of the stack" has been added to the 6.5-stable tree

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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;
 		}



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