m68k: Move signal frame following exception on 68020/030

On 68030/020, an instruction such as, moveml %a2-%a3/%a5,%sp@- may cause
a stack page fault during instruction execution (i.e. not at an
instruction boundary) and produce a format 0xB exception frame.

In this situation, the value of USP will be unreliable.  If a signal is
to be delivered following the exception, this USP value is used to
calculate the location for a signal frame.  This can result in a
corrupted user stack.

The corruption was detected in dash (actually in glibc) where it showed
up as an intermittent "stack smashing detected" message and crash
following signal delivery for SIGCHLD.

It was hard to reproduce that failure because delivery of the signal
raced with the page fault and because the kernel places an unpredictable
gap of up to 7 bytes between the USP and the signal frame.

A format 0xB exception frame can be produced by a bus error or an
address error.  The 68030 Users Manual says that address errors occur
immediately upon detection during instruction prefetch.  The instruction
pipeline allows prefetch to overlap with other instructions, which means
an address error can arise during the execution of a different
instruction.  So it seems likely that this patch may help in the address
error case also.
Reported-and-tested-by: Stan Johnson <userm57@yahoo.com>
Link: https://lore.kernel.org/all/CAMuHMdW3yD22_ApemzW_6me3adq6A458u1_F0v-1EYwK_62jPA@mail.gmail.com/
Cc: Michael Schmitz <schmitzmic@gmail.com>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Cc: stable@vger.kernel.org
Co-developed-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/9e66262a754fcba50208aa424188896cc52a1dd1.1683365892.git.fthain@linux-m68k.orgSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/kernel/signal.c

@@ -858,11 +858,17 @@ static inline int rt_setup_ucontext(struct ucontext __user *uc, struct pt_regs *
 }
 
 static inline void __user *
-get_sigframe(struct ksignal *ksig, size_t frame_size)
+get_sigframe(struct ksignal *ksig, struct pt_regs *tregs, size_t frame_size)
 {
 	unsigned long usp = sigsp(rdusp(), ksig);
+	unsigned long gap = 0;
 
-	return (void __user *)((usp - frame_size) & -8UL);
+	if (CPU_IS_020_OR_030 && tregs->format == 0xb) {
+		/* USP is unreliable so use worst-case value */
+		gap = 256;
+	}
+
+	return (void __user *)((usp - gap - frame_size) & -8UL);
 }
 
 static int setup_frame(struct ksignal *ksig, sigset_t *set,
@@ -880,7 +886,7 @@ static int setup_frame(struct ksignal *ksig, sigset_t *set,
 		return -EFAULT;
 	}
 
-	frame = get_sigframe(ksig, sizeof(*frame) + fsize);
+	frame = get_sigframe(ksig, tregs, sizeof(*frame) + fsize);
 
 	if (fsize)
 		err |= copy_to_user (frame + 1, regs + 1, fsize);
@@ -952,7 +958,7 @@ static int setup_rt_frame(struct ksignal *ksig, sigset_t *set,
 		return -EFAULT;
 	}
 
-	frame = get_sigframe(ksig, sizeof(*frame));
+	frame = get_sigframe(ksig, tregs, sizeof(*frame));
 
 	if (fsize)
 		err |= copy_to_user (&frame->uc.uc_extra, regs + 1, fsize);