The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 31cd31c9e17ece125aad27259501a2af69ccb020 Gitweb: https://git.kernel.org/tip/31cd31c9e17ece125aad27259501a2af69ccb020 Author: Fushuai Wang wangfushuai@baidu.com AuthorDate: Mon, 11 Aug 2025 11:50:44 -07:00 Committer: Dave Hansen dave.hansen@linux.intel.com CommitterDate: Mon, 11 Aug 2025 13:28:07 -07:00

x86/fpu: Fix NULL dereference in avx512_status()

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

This is because the AVX-512 timestamp code uses x86_task_fpu() but doesn't check it for NULL. CONFIG_X86_DEBUG_FPU addles that function for kernel threads (PF_KTHREAD specifically), making it return NULL.

The point of the warning was to ensure that kernel threads only access task->fpu after going through kernel_fpu_begin()/_end(). Note: all kernel tasks exposed in /proc have a valid task->fpu.

Solution -------- One option is to silence the warning and check for NULL from x86_task_fpu(). However, that warning is fairly fresh and seems like a defense against misuse of the FPU state in kernel threads.

Instead, stop outputting AVX-512_elapsed_ms for kernel threads altogether. The data was garbage anyway because avx512_timestamp is only updated for user threads, not kernel threads.

If anyone ever wants to track kernel thread AVX-512 use, they can come back later and do it properly, separate from this bug fix.

[ dhansen: mostly rewrite changelog ]

Fixes: 22aafe3bcb67 ("x86/fpu: Remove init_task FPU state dependencies, add debugging warning for PF_KTHREAD tasks") Co-developed-by: Sohil Mehta sohil.mehta@intel.com Signed-off-by: Sohil Mehta sohil.mehta@intel.com Signed-off-by: Fushuai Wang wangfushuai@baidu.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Cc: stable@vger.kernel.org Link: https://lore.kernel.org/all/20250811185044.2227268-1-sohil.mehta%40intel.com --- 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 12ed75c..28e4fd6 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