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

[Viktor Malik <vmalik@redhat.com>]

It may happen that mm is already released, which leads to kernel panic.
This adds the NULL check for current->mm, similarly to 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
    [...]

Fixes: 20002ded4d93 ("perf_counter: powerpc: Add callchain support")
Signed-off-by: Viktor Malik <vmalik@redhat.com>
---
 arch/powerpc/perf/callchain_32.c | 3 +++
 arch/powerpc/perf/callchain_64.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/arch/powerpc/perf/callchain_32.c b/arch/powerpc/perf/callchain_32.c
index ddcc2d8aa64a..b46e21679566 100644
--- a/arch/powerpc/perf/callchain_32.c
+++ b/arch/powerpc/perf/callchain_32.c
@@ -144,6 +144,9 @@ void perf_callchain_user_32(struct perf_callchain_entry_ctx *entry,
 	sp = regs->gpr[1];
 	perf_callchain_store(entry, next_ip);
 
+	if (!current->mm)
+		return;
+
 	while (entry->nr < entry->max_stack) {
 		fp = (unsigned int __user *) (unsigned long) sp;
 		if (invalid_user_sp(sp) || read_user_stack_32(fp, &next_sp))
diff --git a/arch/powerpc/perf/callchain_64.c b/arch/powerpc/perf/callchain_64.c
index 115d1c105e8a..eaaadd6fa81b 100644
--- a/arch/powerpc/perf/callchain_64.c
+++ b/arch/powerpc/perf/callchain_64.c
@@ -79,6 +79,9 @@ void perf_callchain_user_64(struct perf_callchain_entry_ctx *entry,
 	sp = regs->gpr[1];
 	perf_callchain_store(entry, next_ip);
 
+	if (!current->mm)
+		return;
+
 	while (entry->nr < entry->max_stack) {
 		fp = (unsigned long __user *) sp;
 		if (invalid_user_sp(sp) || read_user_stack_64(fp, &next_sp))
-- 
2.53.0