Currently, if a tracer changes a syscall nr to __NR_future_enosys,
behavior will differ between kernels that know about
__NR_future_enosys (and return -ENOSYS) and older kernels (which
return the value from pt_regs). This is silly; we should just
return -ENOSYS.
This is unlikely to ever happen on x86 because the return value in
pt_regs starts out as -ENOSYS, but a silly tracer can change that.
Signed-off-by: Andy Lutomirski <l...@amacapital.net>
Cc: Will Drewry <w...@chromium.org>
---
arch/x86/include/asm/syscall.h | 11 +++++++++++
kernel/seccomp.c | 15 +++++++++++++++
2 files changed, 26 insertions(+), 0 deletions(-)
diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index 1ace47b..8191e057 100644
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -70,6 +70,17 @@ static inline void syscall_set_return_value(struct
task_struct *task,
regs->ax = (long) error ?: val;
}
+static inline bool syscall_is_nr_future(struct task_struct *task,
+ struct pt_regs *regs)
+{
+#ifdef CONFIG_IA32_EMULATION
+ if (task_thread_info(task)->status & TS_COMPAT)
+ return syscall_get_nr(task, regs) > __NR_ia32_syscall_max;
+#endif
+
+ return syscall_get_nr(task, regs) > __NR_syscall_max;
+}
+
#ifdef CONFIG_X86_32
static inline void syscall_get_arguments(struct task_struct *task,
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 5af44b5..bd7527d 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -429,6 +429,21 @@ int __secure_computing(int this_syscall)
*/
if (fatal_signal_pending(current))
break;
+
+ if (syscall_is_nr_future(current, regs)) {
+ /*
+ * If the tracer selects a system call that
+ * only future kernels know about, we still need
+ * to execute that syscall by returning
+ * -ENOSYS. (On x86, this only matters if the
+ * tracer changed the return value, which would
+ * be silly, but user code can be silly.)
+ */
+ syscall_set_return_value(current, regs,
+ -ENOSYS, 0);
+ goto skip;
+ }
+
if (syscall_get_nr(current, regs) < 0)
goto skip; /* Explicit request to skip. */
--
1.7.7.6
--
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