Commit 910cd32e552e ("parisc: Fix and enable seccomp filter support")
introduced a regression in ptrace-based syscall tampering: when tracer
changes syscall number to -1, the kernel fails to initialize %r28 with
-ENOSYS and subsequently fails to return the error code of the failed
syscall to userspace.
This erroneous behaviour could be observed with a simple strace syscall
fault injection command which is expected to print something like this:
$ strace -a0 -ewrite -einject=write:error=enospc echo hello
write(1, "hello\n", 6) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "echo: ", 6) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "write error", 11) = -1 ENOSPC (No space left on device) (INJECTED)
write(2, "\n", 1) = -1 ENOSPC (No space left on device) (INJECTED)
+++ exited with 1 +++
After commit 910cd32e552ea09caa89cdbe328e468979b030dd it loops printing
something like this instead:
write(1, "hello\n", 6../strace: Failed to tamper with process 12345:
unexpectedly got no error (return value 0, error 0)
) = 0 (INJECTED)
This bug was found by strace test suite.
Fixes: 910cd32e552e ("parisc: Fix and enable seccomp filter support")
Cc: sta...@vger.kernel.org # v4.5+
Signed-off-by: Dmitry V. Levin <l...@altlinux.org>
---
v2: Updated comments.
Set gr[28] to -ENOSYS after tracehook_report_syscall_entry() invocation
because setting of syscall return code by tracer on entering syscall
shall not affect the syscall return code.
N.B. I have no parisc box to test the patch.
arch/parisc/kernel/ptrace.c | 28 +++++++++++++++++++++-------
1 file changed, 21 insertions(+), 7 deletions(-)
diff --git a/arch/parisc/kernel/ptrace.c b/arch/parisc/kernel/ptrace.c
index 2582df1c529b..be4d6a279b12 100644
--- a/arch/parisc/kernel/ptrace.c
+++ b/arch/parisc/kernel/ptrace.c
@@ -308,15 +308,29 @@ long compat_arch_ptrace(struct task_struct *child,
compat_long_t request,
long do_syscall_trace_enter(struct pt_regs *regs)
{
- if (test_thread_flag(TIF_SYSCALL_TRACE) &&
- tracehook_report_syscall_entry(regs)) {
+ if (test_thread_flag(TIF_SYSCALL_TRACE)) {
+ int rc = tracehook_report_syscall_entry(regs);
+
/*
- * Tracing decided this syscall should not happen or the
- * debugger stored an invalid system call number. Skip
- * the system call and the system call restart handling.
+ * As tracesys_next does not set %r28 to -ENOSYS
+ * when %r20 is set to -1, initialize it here.
*/
- regs->gr[20] = -1UL;
- goto out;
+ regs->gr[28] = -ENOSYS;
+
+ if (rc) {
+ /*
+ * A nonzero return code from
+ * tracehook_report_syscall_entry() tells us
+ * to prevent the syscall execution. Skip
+ * the syscall call and the syscall restart handling.
+ *
+ * Note that the tracer may also just change
+ * regs->gr[20] to an invalid syscall number,
+ * that is handled by tracesys_next.
+ */
+ regs->gr[20] = -1UL;
+ return -1;
+ }
}
/* Do the secure computing check after ptrace. */
--
ldv
