4.9-stable review patch. If anyone has any objections, please let me know.
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From: Dmitry V. Levin ldv@altlinux.org
commit b7dc5a071ddf69c0350396b203cba32fe5bab510 upstream.
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: stable@vger.kernel.org # v4.5+ Signed-off-by: Dmitry V. Levin ldv@altlinux.org Tested-by: Helge Deller deller@gmx.de Signed-off-by: Helge Deller deller@gmx.de Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/parisc/kernel/ptrace.c | 29 +++++++++++++++++++++-------- 1 file changed, 21 insertions(+), 8 deletions(-)
--- a/arch/parisc/kernel/ptrace.c +++ b/arch/parisc/kernel/ptrace.c @@ -311,15 +311,29 @@ long compat_arch_ptrace(struct task_stru
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. */ @@ -343,7 +357,6 @@ long do_syscall_trace_enter(struct pt_re regs->gr[24] & 0xffffffff, regs->gr[23] & 0xffffffff);
-out: /* * Sign extend the syscall number to 64bit since it may have been * modified by a compat ptrace call