On Wed, Nov 29, 2017 at 03:21:14PM +0000, Maciej W. Rozycki wrote:
Fix a commit d614fd58a283 ("mips/ptrace: Preserve previous registers for short regset write") bug and allow the last register requested with a ptrace(2) PTRACE_SETREGSET call to be partially written if supplied this way by the caller, like with other register sets.
Cc: stable@vger.kernel.org # v4.11+ Fixes: d614fd58a283 ("mips/ptrace: Preserve previous registers for short regset write") Signed-off-by: Maciej W. Rozycki macro@mips.com
arch/mips/kernel/ptrace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
linux-mips-nt-prfpreg-count.diff Index: linux-sfr-test/arch/mips/kernel/ptrace.c =================================================================== --- linux-sfr-test.orig/arch/mips/kernel/ptrace.c 2017-11-21 22:12:00.000000000 +0000 +++ linux-sfr-test/arch/mips/kernel/ptrace.c 2017-11-21 22:13:13.471970000 +0000 @@ -484,7 +484,7 @@ static int fpr_set_msa(struct task_struc int err; BUILD_BUG_ON(sizeof(fpr_val) != sizeof(elf_fpreg_t));
- for (i = 0; i < NUM_FPU_REGS && *count >= sizeof(elf_fpreg_t); i++) {
for (i = 0; i < NUM_FPU_REGS && *count > 0; i++) {
err = user_regset_copyin(pos, count, kbuf, ubuf, &fpr_val, i * sizeof(elf_fpreg_t), (i + 1) * sizeof(elf_fpreg_t));
But mips*_regsets[REGSET_FPR].size == sizeof(elf_fpreg_t), linux/kernel/regset.c:ptrace_regset() polices iov_len % regset->size == 0, and each user_regset_copyout() call here transfers sizeof(elf_fpreg_t) bytes, decrementing *count by that amount unless something goest wrong in which case we return.
So how do we end up with *count > 0 && *count < sizeof(elf_fpreg_t) here?
If we can't end up with that, then this patch doesn't change ABI- observable behaviour, unless I've missed something.
If we can end up with that somehow, then this patch reintroduces the issue d614fd58a283 aims to fix, whereby fpr_val can contain uninitialised kernel stack which userspace can then obtain via PTRACE_GETREGSET.
Cheers ---Dave