From: Finn Thain fthain@linux-m68k.org

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_62... 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/9e66262a754fcba50208aa424188896cc52a1dd1.168336589... Signed-off-by: Geert Uytterhoeven geert@linux-m68k.org --- arch/m68k/kernel/signal.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/m68k/kernel/signal.c b/arch/m68k/kernel/signal.c index 8fb8ee804b3a..de7c1bde62bc 100644 --- a/arch/m68k/kernel/signal.c +++ b/arch/m68k/kernel/signal.c @@ -808,11 +808,17 @@ 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, @@ -830,7 +836,7 @@ 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); @@ -903,7 +909,7 @@ 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);