This is a note to let you know that I've just added the patch titled
x86/fpu: Drop fpregs lock before inheriting FPU permissions
to the 6.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-fpu-drop-fpregs-lock-before-inheriting-fpu-permissions.patch
and it can be found in the queue-6.0 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 36b038791e1e2baea892e9276588815fd14894b4 Mon Sep 17 00:00:00 2001
From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Date: Thu, 10 Nov 2022 12:44:00 +0000
Subject: x86/fpu: Drop fpregs lock before inheriting FPU permissions
From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
commit 36b038791e1e2baea892e9276588815fd14894b4 upstream.
Mike Galbraith reported the following against an old fork of preempt-rt
but the same issue also applies to the current preempt-rt tree.
BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:46
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: systemd
preempt_count: 1, expected: 0
RCU nest depth: 0, expected: 0
Preemption disabled at:
fpu_clone
CPU: 6 PID: 1 Comm: systemd Tainted: G E (unreleased)
Call Trace:
<TASK>
dump_stack_lvl
? fpu_clone
__might_resched
rt_spin_lock
fpu_clone
? copy_thread
? copy_process
? shmem_alloc_inode
? kmem_cache_alloc
? kernel_clone
? __do_sys_clone
? do_syscall_64
? __x64_sys_rt_sigprocmask
? syscall_exit_to_user_mode
? do_syscall_64
? syscall_exit_to_user_mode
? do_syscall_64
? syscall_exit_to_user_mode
? do_syscall_64
? exc_page_fault
? entry_SYSCALL_64_after_hwframe
</TASK>
Mike says:
The splat comes from fpu_inherit_perms() being called under fpregs_lock(),
and us reaching the spin_lock_irq() therein due to fpu_state_size_dynamic()
returning true despite static key __fpu_state_size_dynamic having never
been enabled.
Mike's assessment looks correct. fpregs_lock on a PREEMPT_RT kernel disables
preemption so calling spin_lock_irq() in fpu_inherit_perms() is unsafe. This
problem exists since commit
9e798e9aa14c ("x86/fpu: Prepare fpu_clone() for dynamically enabled features").
Even though the original bug report should not have enabled the paths at
all, the bug still exists.
fpregs_lock is necessary when editing the FPU registers or a task's FP
state but it is not necessary for fpu_inherit_perms(). The only write
of any FP state in fpu_inherit_perms() is for the new child which is
not running yet and cannot context switch or be borrowed by a kernel
thread yet. Hence, fpregs_lock is not protecting anything in the new
child until clone() completes and can be dropped earlier. The siglock
still needs to be acquired by fpu_inherit_perms() as the read of the
parent's permissions has to be serialised.
[ bp: Cleanup splat. ]
Fixes: 9e798e9aa14c ("x86/fpu: Prepare fpu_clone() for dynamically enabled features")
Reported-by: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20221110124400.zgymc2lnwqjukgfh@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kernel/fpu/core.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 3b28c5b25e12..d00db56a8868 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -605,9 +605,9 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal)
if (test_thread_flag(TIF_NEED_FPU_LOAD))
fpregs_restore_userregs();
save_fpregs_to_fpstate(dst_fpu);
+ fpregs_unlock();
if (!(clone_flags & CLONE_THREAD))
fpu_inherit_perms(dst_fpu);
- fpregs_unlock();
/*
* Children never inherit PASID state.
--
2.38.1
Patches currently in stable-queue which might be from mgorman@xxxxxxxxxxxxxxxxxxx are
queue-6.0/x86-fpu-drop-fpregs-lock-before-inheriting-fpu-permissions.patch
