[PATCH] perf: Fix oops when kthread execs user process

[Young Xiao]

When a kthread calls call_usermodehelper() the steps are:
  1. allocate current->mm
  2. load_elf_binary()
  3. populate current->thread.regs

While doing this, interrupts are not disabled. If there is a perf
interrupt in the middle of this process (i.e. step 1 has completed
but not yet reached to step 3) and if perf tries to read userspace
regs, kernel oops.

Fix it by setting abi to PERF_SAMPLE_REGS_ABI_NONE when userspace
pt_regs are not set.

See commit bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs
user process") for details.

Signed-off-by: Young Xiao <92siuy...@gmail.com>
---
 arch/arm/kernel/perf_regs.c   | 3 ++-
 arch/arm64/kernel/perf_regs.c | 3 ++-
 arch/x86/kernel/perf_regs.c   | 3 ++-
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/arm/kernel/perf_regs.c b/arch/arm/kernel/perf_regs.c
index 05fe92a..78ee29a 100644
--- a/arch/arm/kernel/perf_regs.c
+++ b/arch/arm/kernel/perf_regs.c
@@ -36,5 +36,6 @@ void perf_get_regs_user(struct perf_regs *regs_user,
                        struct pt_regs *regs_user_copy)
 {
        regs_user->regs = task_pt_regs(current);
-       regs_user->abi = perf_reg_abi(current);
+       regs_user->abi = (regs_user->regs) ? perf_reg_abi(current) :
+                        PERF_SAMPLE_REGS_ABI_NONE;
 }
diff --git a/arch/arm64/kernel/perf_regs.c b/arch/arm64/kernel/perf_regs.c
index 0bbac61..ac19d19 100644
--- a/arch/arm64/kernel/perf_regs.c
+++ b/arch/arm64/kernel/perf_regs.c
@@ -58,5 +58,6 @@ void perf_get_regs_user(struct perf_regs *regs_user,
                        struct pt_regs *regs_user_copy)
 {
        regs_user->regs = task_pt_regs(current);
-       regs_user->abi = perf_reg_abi(current);
+       regs_user->abi = (regs_user->regs) ? perf_reg_abi(current) :
+                        PERF_SAMPLE_REGS_ABI_NONE;
 }
diff --git a/arch/x86/kernel/perf_regs.c b/arch/x86/kernel/perf_regs.c
index 07c30ee..fa79d6d 100644
--- a/arch/x86/kernel/perf_regs.c
+++ b/arch/x86/kernel/perf_regs.c
@@ -102,7 +102,8 @@ void perf_get_regs_user(struct perf_regs *regs_user,
                        struct pt_regs *regs_user_copy)
 {
        regs_user->regs = task_pt_regs(current);
-       regs_user->abi = perf_reg_abi(current);
+       regs_user->abi = (regs_user->regs) ? perf_reg_abi(current) :
+                        PERF_SAMPLE_REGS_ABI_NONE;
 }
 #else /* CONFIG_X86_64 */
 #define REG_NOSUPPORT ((1ULL << PERF_REG_X86_DS) | \
-- 
2.7.4