CVE-2025-38261

In the Linux kernel, the following vulnerability has been resolved:

riscv: save the SR_SUM status over switches

When threads/tasks are switched we need to ensure the old execution's

SR_SUM state is saved and the new thread has the old SR_SUM state

restored.

The issue was seen under heavy load especially with the syz-stress tool

running, with crashes as follows in schedule_tail:

Unable to handle kernel access to user memory without uaccess routines

at virtual address 000000002749f0d0

Oops [#1]

Modules linked in:

CPU: 1 PID: 4875 Comm: syz-executor.0 Not tainted

5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0

Hardware name: riscv-virtio,qemu (DT)

epc : schedule_tail+0x72/0xb2 kernel/sched/core.c:4264

ra : task_pid_vnr include/linux/sched.h:1421 [inline]

ra : schedule_tail+0x70/0xb2 kernel/sched/core.c:4264

epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp : ffffffe025d17ec0

gp : ffffffe005d25378 tp : ffffffe00f0d0000 t0 : 0000000000000000

t1 : 0000000000000001 t2 : 00000000000f4240 s0 : ffffffe025d17ee0

s1 : 000000002749f0d0 a0 : 000000000000002a a1 : 0000000000000003

a2 : 1ffffffc0cfac500 a3 : ffffffe0000c80cc a4 : 5ae9db91c19bbe00

a5 : 0000000000000000 a6 : 0000000000f00000 a7 : ffffffe000082eba

s2 : 0000000000040000 s3 : ffffffe00eef96c0 s4 : ffffffe022c77fe0

s5 : 0000000000004000 s6 : ffffffe067d74e00 s7 : ffffffe067d74850

s8 : ffffffe067d73e18 s9 : ffffffe067d74e00 s10: ffffffe00eef96e8

s11: 000000ae6cdf8368 t3 : 5ae9db91c19bbe00 t4 : ffffffc4043cafb2

t5 : ffffffc4043cafba t6 : 0000000000040000

status: 0000000000000120 badaddr: 000000002749f0d0 cause:

000000000000000f

Call Trace:

[<ffffffe00008c8b0>] schedule_tail+0x72/0xb2 kernel/sched/core.c:4264

[<ffffffe000005570>] ret_from_exception+0x0/0x14

Dumping ftrace buffer:

(ftrace buffer empty)

---[ end trace b5f8f9231dc87dda ]---

The issue comes from the put_user() in schedule_tail

(kernel/sched/core.c) doing the following:

asmlinkage __visible void schedule_tail(struct task_struct *prev)

{

...

if (current->set_child_tid)

put_user(task_pid_vnr(current), current->set_child_tid);

...

}

the put_user() macro causes the code sequence to come out as follows:

1: __enable_user_access()

2: reg = task_pid_vnr(current);

3: *current->set_child_tid = reg;

4: __disable_user_access()

The problem is that we may have a sleeping function as argument which

could clear SR_SUM causing the panic above. This was fixed by

evaluating the argument of the put_user() macro outside the user-enabled

section in commit 285a76bb2cf5 ("riscv: evaluate put_user() arg before

enabling user access")"

In order for riscv to take advantage of unsafe_get/put_XXX() macros and

to avoid the same issue we had with put_user() and sleeping functions we

must ensure code flow can go through switch_to() from within a region of

code with SR_SUM enabled and come back with SR_SUM still enabled. This

patch addresses the problem allowing future work to enable full use of

unsafe_get/put_XXX() macros without needing to take a CSR bit flip cost

on every access. Make switch_to() save and restore SR_SUM.