[PATCH v4] x86/fpu: Fix NULL dereference in avx512_status()

[Sohil Mehta]

From: Fushuai Wang <wangfushuai@xxxxxxxxx>

Problem
-------
With CONFIG_X86_DEBUG_FPU enabled, reading /proc/[kthread]/arch_status
causes a kernel NULL pointer dereference.

Kernel threads aren't expected to access the FPU state directly. Kernel
usage of FPU registers is contained within kernel_fpu_begin()/_end()
sections.

However, to report AVX-512 usage, the avx512_timestamp variable within
struct fpu needs to be accessed, which triggers a warning in
x86_task_fpu().

For Kthreads:
  proc_pid_arch_status()
    avx512_status()
      x86_task_fpu() => Warning and returns NULL
      x86_task_fpu()->avx512_timestamp => NULL dereference

The warning is a false alarm since the access isn't intended for
modifying the FPU state. All kernel threads (except the init_task) have
a "struct fpu" with an avx512_timestamp variable that is valid to
access. Also, the init_task (PID 0) never follows this path since it is
not exposed in /proc.

Solution
--------
One option is to get rid of the warning in x86_task_fpu() for kernel
threads. However, that warning was recently added and might be useful to
catch any potential misuse of the FPU state in kernel threads.

A better option is to avoid the access altogether. The kernel does not
track AVX-512 usage for kernel threads.
save_fpregs_to_fpstate()->update_avx_timestamp() is never invoked for
kernel threads, so avx512_timestamp is always guaranteed to be 0.

Also, the legacy behavior of reporting "AVX512_elapsed_ms: -1", which
signifies "no AVX-512 usage", is misleading. The kernel usage just isn't
tracked.

For now, update the ABI for kernel threads and do not report AVX-512
usage for them. Reading /proc/[kthread]/arch_status would display no
AVX-512 information. This avoids the NULL dereference as well as the
misleading report.

Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Fixes: 22aafe3bcb67 ("x86/fpu: Remove init_task FPU state dependencies, add debugging warning for PF_KTHREAD tasks")
Cc: <stable@xxxxxxxxxxxxxxx> # v6.15+
Signed-off-by: Fushuai Wang <wangfushuai@xxxxxxxxx>
Co-developed-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
---
v4:
 - No significant change, minor wording improvements.

v3: https://lore.kernel.org/lkml/20250724013422.307954-1-sohil.mehta@xxxxxxxxx/
 - Do not report anything for kernel threads. (DaveH)
 - Make the commit message more precise.

v2:
 - Avoid making the fix dependent on CONFIG_X86_DEBUG_FPU.
 - Include PF_USER_WORKER in the kernel thread check.
 - Update commit message for clarity.
---
 arch/x86/kernel/fpu/xstate.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 12ed75c1b567..28e4fd65c9da 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1881,19 +1881,20 @@ long fpu_xstate_prctl(int option, unsigned long arg2)
 #ifdef CONFIG_PROC_PID_ARCH_STATUS
 /*
  * Report the amount of time elapsed in millisecond since last AVX512
- * use in the task.
+ * use in the task. Report -1 if no AVX-512 usage.
  */
 static void avx512_status(struct seq_file *m, struct task_struct *task)
 {
-	unsigned long timestamp = READ_ONCE(x86_task_fpu(task)->avx512_timestamp);
-	long delta;
+	unsigned long timestamp;
+	long delta = -1;
 
-	if (!timestamp) {
-		/*
-		 * Report -1 if no AVX512 usage
-		 */
-		delta = -1;
-	} else {
+	/* AVX-512 usage is not tracked for kernel threads. Don't report anything. */
+	if (task->flags & (PF_KTHREAD | PF_USER_WORKER))
+		return;
+
+	timestamp = READ_ONCE(x86_task_fpu(task)->avx512_timestamp);
+
+	if (timestamp) {
 		delta = (long)(jiffies - timestamp);
 		/*
 		 * Cap to LONG_MAX if time difference > LONG_MAX
-- 
2.43.0