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powerpc, perf: Check that current->mm is alive before getting user callchain

It may happen that mm is already released, which leads to kernel panic.
This adds the NULL check for current->mm, similarly to
commit 20afc60f892d ("x86, perf: Check that current->mm is alive before getting user callchain").

I was getting this panic when running a profiling BPF program
(profile.py from bcc-tools):

[26215.051935] Kernel attempted to read user page (588) - exploit attempt? (uid: 0)
[26215.051950] BUG: Kernel NULL pointer dereference on read at 0x00000588
[26215.051952] Faulting instruction address: 0xc00000000020fac0
[26215.051957] Oops: Kernel access of bad area, sig: 11 [#1]
[...]
[26215.052049] Call Trace:
[26215.052050] [c000000061da6d30] [c00000000020fc10] perf_callchain_user_64+0x2d0/0x490 (unreliable)
[26215.052054] [c000000061da6dc0] [c00000000020f92c] perf_callchain_user+0x1c/0x30
[26215.052057] [c000000061da6de0] [c0000000005ab2a0] get_perf_callchain+0x100/0x360
[26215.052063] [c000000061da6e70] [c000000000573bc8] bpf_get_stackid+0x88/0xf0
[26215.052067] [c000000061da6ea0] [c008000000042258] bpf_prog_16d4ab9ab662f669_do_perf_event+0xf8/0x274
[...]

In addition, move storing the top-level stack entry to generic
perf_callchain_user to make sure the top-evel entry is always captured,
even if current->mm is NULL.

Fixes: 20002ded4d93 ("perf_counter: powerpc: Add callchain support")
Signed-off-by: Viktor Malik <vmalik@redhat.com>
Tested-by: Qiao Zhao <qzhao@redhat.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
[Maddy: fixed message to avoid checkpatch format style error]
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260309144045.169427-1-vmalik@redhat.com

authored by

Viktor Malik and committed by
Madhavan Srinivasan e9bbfb4b 0a8321dd

+5 -2
+5
arch/powerpc/perf/callchain.c
··· 103 103 void 104 104 perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs) 105 105 { 106 + perf_callchain_store(entry, perf_arch_instruction_pointer(regs)); 107 + 108 + if (!current->mm) 109 + return; 110 + 106 111 if (!is_32bit_task()) 107 112 perf_callchain_user_64(entry, regs); 108 113 else
-1
arch/powerpc/perf/callchain_32.c
··· 142 142 next_ip = perf_arch_instruction_pointer(regs); 143 143 lr = regs->link; 144 144 sp = regs->gpr[1]; 145 - perf_callchain_store(entry, next_ip); 146 145 147 146 while (entry->nr < entry->max_stack) { 148 147 fp = (unsigned int __user *) (unsigned long) sp;
-1
arch/powerpc/perf/callchain_64.c
··· 77 77 next_ip = perf_arch_instruction_pointer(regs); 78 78 lr = regs->link; 79 79 sp = regs->gpr[1]; 80 - perf_callchain_store(entry, next_ip); 81 80 82 81 while (entry->nr < entry->max_stack) { 83 82 fp = (unsigned long __user *) sp;