CVE-2023-52761

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

riscv: VMAP_STACK overflow detection thread-safe

commit 31da94c25aea ("riscv: add VMAP_STACK overflow detection") added

support for CONFIG_VMAP_STACK. If overflow is detected, CPU switches to

`shadow_stack` temporarily before switching finally to per-cpu

`overflow_stack`.

If two CPUs/harts are racing and end up in over flowing kernel stack, one

or both will end up corrupting each other state because `shadow_stack` is

not per-cpu. This patch optimizes per-cpu overflow stack switch by

directly picking per-cpu `overflow_stack` and gets rid of `shadow_stack`.

Following are the changes in this patch

- Defines an asm macro to obtain per-cpu symbols in destination

register.

- In entry.S, when overflow is detected, per-cpu overflow stack is

located using per-cpu asm macro. Computing per-cpu symbol requires

a temporary register. x31 is saved away into CSR_SCRATCH

(CSR_SCRATCH is anyways zero since we're in kernel).

Please see Links for additional relevant disccussion and alternative

solution.

Tested by `echo EXHAUST_STACK > /sys/kernel/debug/provoke-crash/DIRECT`

Kernel crash log below

Insufficient stack space to handle exception!/debug/provoke-crash/DIRECT

Task stack: [0xff20000010a98000..0xff20000010a9c000]

Overflow stack: [0xff600001f7d98370..0xff600001f7d99370]

CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34

Hardware name: riscv-virtio,qemu (DT)

epc : __memset+0x60/0xfc

ra : recursive_loop+0x48/0xc6 [lkdtm]

epc : ffffffff808de0e4 ra : ffffffff0163a752 sp : ff20000010a97e80

gp : ffffffff815c0330 tp : ff600000820ea280 t0 : ff20000010a97e88

t1 : 000000000000002e t2 : 3233206874706564 s0 : ff20000010a982b0

s1 : 0000000000000012 a0 : ff20000010a97e88 a1 : 0000000000000000

a2 : 0000000000000400 a3 : ff20000010a98288 a4 : 0000000000000000

a5 : 0000000000000000 a6 : fffffffffffe43f0 a7 : 00007fffffffffff

s2 : ff20000010a97e88 s3 : ffffffff01644680 s4 : ff20000010a9be90

s5 : ff600000842ba6c0 s6 : 00aaaaaac29e42b0 s7 : 00fffffff0aa3684

s8 : 00aaaaaac2978040 s9 : 0000000000000065 s10: 00ffffff8a7cad10

s11: 00ffffff8a76a4e0 t3 : ffffffff815dbaf4 t4 : ffffffff815dbaf4

t5 : ffffffff815dbab8 t6 : ff20000010a9bb48

status: 0000000200000120 badaddr: ff20000010a97e88 cause: 000000000000000f

Kernel panic - not syncing: Kernel stack overflow

CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34

Hardware name: riscv-virtio,qemu (DT)

Call Trace:

[<ffffffff80006754>] dump_backtrace+0x30/0x38

[<ffffffff808de798>] show_stack+0x40/0x4c

[<ffffffff808ea2a8>] dump_stack_lvl+0x44/0x5c

[<ffffffff808ea2d8>] dump_stack+0x18/0x20

[<ffffffff808dec06>] panic+0x126/0x2fe

[<ffffffff800065ea>] walk_stackframe+0x0/0xf0

[<ffffffff0163a752>] recursive_loop+0x48/0xc6 [lkdtm]

SMP: stopping secondary CPUs

---[ end Kernel panic - not syncing: Kernel stack overflow ]---