Commit b7dc5a07 authored by Dmitry V. Levin's avatar Dmitry V. Levin Committed by Helge Deller

parisc: Fix ptrace syscall number modification

Commit 910cd32e ("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 910cd32e 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: 910cd32e ("parisc: Fix and enable seccomp filter support")
Cc: [email protected] # v4.5+
Signed-off-by: default avatarDmitry V. Levin <[email protected]>
Tested-by: default avatarHelge Deller <[email protected]>
Signed-off-by: default avatarHelge Deller <[email protected]>
parent f6163d67
......@@ -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. */
......@@ -340,7 +354,6 @@ long do_syscall_trace_enter(struct pt_regs *regs)
regs->gr[24] & 0xffffffff,
regs->gr[23] & 0xffffffff);
* Sign extend the syscall number to 64bit since it may have been
* modified by a compat ptrace call
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