From: Ali Ahmet Memis Date: Fri, 7 Aug 2026 23:42:30 +0000 (+0000) Subject: openrisc: signal: do not restore privileged SR bits on sigreturn X-Git-Tag: v7.2~11^2 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=32ef1b30ad736519f7a207bcc2986f3d4129d972;p=thirdparty%2Fkernel%2Flinux.git openrisc: signal: do not restore privileged SR bits on sigreturn restore_sigcontext() copies the whole supervision register (SR) from the signal frame and only clears SPR_SR_SM before the value is reloaded into the hardware SR (through ESR and l.rfe) on the return to user space. All other SR bits are left under user control. An unprivileged task can thus return from a signal handler through a crafted sigframe that clears SPR_SR_DME. With the data MMU disabled the CPU performs no translation or protection on data accesses, so the task gains read and write access to arbitrary physical memory, a local privilege escalation. SPR_SR_IME, SPR_SR_SUMRA, SPR_SR_LEE, SPR_SR_EPH and the cache-enable bits are exposed the same way. The ptrace GPR regset already refuses any change to SR for exactly this reason. Restore only the arithmetic flag bits (F, CY, OV) from the signal frame and take every privileged control bit from the SR the kernel saved on signal entry. Verified with qemu-system-or1k -M or1k-sim: before this change an unprivileged PoC clears SPR_SR_DME in rt_sigreturn and writes a marker to physical address 0x03000000 (beyond the kernel's mem=32M); afterwards the same PoC receives SIGSEGV and physical memory is unchanged. Fixes: ac689eb7f9d4 ("OpenRISC: Signal handling") Cc: stable@vger.kernel.org Signed-off-by: Ali Ahmet Memis Signed-off-by: Stafford Horne --- diff --git a/arch/openrisc/include/asm/processor.h b/arch/openrisc/include/asm/processor.h index 3ff893a67c13b..ae43fe79b2b5c 100644 --- a/arch/openrisc/include/asm/processor.h +++ b/arch/openrisc/include/asm/processor.h @@ -26,6 +26,8 @@ | SPR_SR_DCE | SPR_SR_SM) #define USER_SR (SPR_SR_DME | SPR_SR_IME | SPR_SR_ICE \ | SPR_SR_DCE | SPR_SR_IEE | SPR_SR_TEE) +/* SR bits user space may change via sigreturn, the rest stay kernel owned */ +#define SPR_SR_USER_MASK (SPR_SR_F | SPR_SR_CY | SPR_SR_OV) /* * User space process size. This is hardcoded into a few places, diff --git a/arch/openrisc/kernel/signal.c b/arch/openrisc/kernel/signal.c index f70a13ee05936..2be5af3b98323 100644 --- a/arch/openrisc/kernel/signal.c +++ b/arch/openrisc/kernel/signal.c @@ -74,6 +74,7 @@ static long save_fp_state(struct sigcontext __user *sc) static int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc) { + unsigned long old_sr = regs->sr; int err = 0; /* Always make any pending restarted system calls return -EINTR */ @@ -89,8 +90,8 @@ static int restore_sigcontext(struct pt_regs *regs, err |= __copy_from_user(®s->sr, &sc->regs.sr, sizeof(unsigned long)); err |= restore_fp_state(sc); - /* make sure the SM-bit is cleared so user-mode cannot fool us */ - regs->sr &= ~SPR_SR_SM; + /* keep the privileged SR bits kernel owned, restore only user flags */ + regs->sr = (old_sr & ~SPR_SR_USER_MASK) | (regs->sr & SPR_SR_USER_MASK); regs->orig_gpr11 = -1; /* Avoid syscall restart checks */