[PATCH 3.5 2/2] seccomp: Future-proof against silly tracers

[Andy Lutomirski]

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 <luto@xxxxxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
---
 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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