[PATCH] sh: prevent ptrace from changing privileged SR bits

PTRACE_POKEUSR writes every word in struct pt_regs, including SR, without
filtering privileged bits. The general register regset setter exposes the
same write through PTRACE_SETREGS and PTRACE_SETREGSET.

The exception return path restores the saved SR with rte after adjusting
only the interrupt mask. An unprivileged same-UID tracer can set SR.MD and
make its tracee resume at a user address in privileged mode.

Preserve the saved non-user SR bits in both write paths and accept only
the bits in SR_USER_MASK. Recompose SR after user_regset_copyin() even on
error because the copy may have updated a prefix of the register set.

Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <[email protected]>
---
 arch/sh/kernel/ptrace_32.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/sh/kernel/ptrace_32.c b/arch/sh/kernel/ptrace_32.c
index 8794081483fb..bb69c35cb9a7 100644
--- a/arch/sh/kernel/ptrace_32.c
+++ b/arch/sh/kernel/ptrace_32.c
@@ -146,6 +146,7 @@ static int genregs_set(struct task_struct *target,
 		       const void *kbuf, const void __user *ubuf)
 {
 	struct pt_regs *regs = task_pt_regs(target);
+	unsigned long sr = regs->sr & ~SR_USER_MASK;
 	int ret;
 
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
@@ -159,6 +160,7 @@ static int genregs_set(struct task_struct *target,
 	if (!ret)
 		user_regset_copyin_ignore(&pos, &count, &kbuf, &ubuf,
 					  sizeof(struct pt_regs), -1);
+	regs->sr = (regs->sr & SR_USER_MASK) | sr;
 
 	return ret;
 }
@@ -391,6 +393,11 @@ long arch_ptrace(struct task_struct *child, long request,
 		    addr > sizeof(struct user) - 3)
 			break;
 
+		if (addr == offsetof(struct pt_regs, sr)) {
+			unsigned long sr = get_stack_long(child, addr);
+
+			data = (data & SR_USER_MASK) | (sr & ~SR_USER_MASK);
+		}
 		if (addr < sizeof(struct pt_regs))
 			ret = put_stack_long(child, addr, data);
 		else if (addr >= offsetof(struct user, fpu) &&
-- 
2.47.3