[Linux-kernel-mentees] [PATCH v3] ptrace: Prevent kernel-infoleak in ptrace_get_syscall_info()

[Peilin Ye]

ptrace_get_syscall_info() is potentially copying uninitialized stack
memory to userspace, since the compiler may leave a 3-byte hole near the
beginning of `info`. Fix it by adding a padding field to `struct
ptrace_syscall_info`.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 201766a20e30 ("ptrace: add PTRACE_GET_SYSCALL_INFO request")
Suggested-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Peilin Ye <yepeilin.cs@xxxxxxxxx>
---
Change in v3:
    - Remove unnecessary `__aligned__` attribute. (Suggested by
      Dmitry V. Levin <ldv@xxxxxxxxxxxx>)

Change in v2:
    - Add a padding field to `struct ptrace_syscall_info`, instead of
      doing memset() on `info`. (Suggested by Dmitry V. Levin
      <ldv@xxxxxxxxxxxx>)

Reference: https://lwn.net/Articles/417989/

$ # before:
$ pahole -C "ptrace_syscall_info" kernel/ptrace.o
struct ptrace_syscall_info {
	__u8                       op;                   /*     0     1 */

	/* XXX 3 bytes hole, try to pack */

	__u32                      arch __attribute__((__aligned__(4))); /*     4     4 */
	__u64                      instruction_pointer;  /*     8     8 */
	__u64                      stack_pointer;        /*    16     8 */
	union {
		struct {
			__u64      nr;                   /*    24     8 */
			__u64      args[6];              /*    32    48 */
		} entry;                                 /*    24    56 */
		struct {
			__s64      rval;                 /*    24     8 */
			__u8       is_error;             /*    32     1 */
		} exit;                                  /*    24    16 */
		struct {
			__u64      nr;                   /*    24     8 */
			__u64      args[6];              /*    32    48 */
			/* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
			__u32      ret_data;             /*    80     4 */
		} seccomp;                               /*    24    64 */
	};                                               /*    24    64 */

	/* size: 88, cachelines: 2, members: 5 */
	/* sum members: 85, holes: 1, sum holes: 3 */
	/* forced alignments: 1, forced holes: 1, sum forced holes: 3 */
	/* last cacheline: 24 bytes */
} __attribute__((__aligned__(8)));
$
$ # after:
$ pahole -C "ptrace_syscall_info" kernel/ptrace.o
struct ptrace_syscall_info {
	__u8                       op;                   /*     0     1 */
	__u8                       pad[3];               /*     1     3 */
	__u32                      arch;                 /*     4     4 */
	__u64                      instruction_pointer;  /*     8     8 */
	__u64                      stack_pointer;        /*    16     8 */
	union {
		struct {
			__u64      nr;                   /*    24     8 */
			__u64      args[6];              /*    32    48 */
		} entry;                                 /*    24    56 */
		struct {
			__s64      rval;                 /*    24     8 */
			__u8       is_error;             /*    32     1 */
		} exit;                                  /*    24    16 */
		struct {
			__u64      nr;                   /*    24     8 */
			__u64      args[6];              /*    32    48 */
			/* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
			__u32      ret_data;             /*    80     4 */
		} seccomp;                               /*    24    64 */
	};                                               /*    24    64 */

	/* size: 88, cachelines: 2, members: 6 */
	/* last cacheline: 24 bytes */
};
$ _

 include/uapi/linux/ptrace.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
index a71b6e3b03eb..83ee45fa634b 100644
--- a/include/uapi/linux/ptrace.h
+++ b/include/uapi/linux/ptrace.h
@@ -81,7 +81,8 @@ struct seccomp_metadata {
 
 struct ptrace_syscall_info {
 	__u8 op;	/* PTRACE_SYSCALL_INFO_* */
-	__u32 arch __attribute__((__aligned__(sizeof(__u32))));
+	__u8 pad[3];
+	__u32 arch;
 	__u64 instruction_pointer;
 	__u64 stack_pointer;
 	union {
-- 
2.25.1