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From:   Peilin Ye <yepeilin.cs@gmail.com>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Peilin Ye <yepeilin.cs@gmail.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
        Elvira Khabirova <lineprinter@altlinux.org>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel-mentees@lists.linuxfoundation.org,
        linux-kernel@vger.kernel.org
Subject: [Linux-kernel-mentees] [PATCH v3] ptrace: Prevent kernel-infoleak in ptrace_get_syscall_info()
Date:   Sat,  1 Aug 2020 11:20:44 -0400
Message-Id: <20200801152044.230416-1-yepeilin.cs@gmail.com>
In-Reply-To: <20200801020841.227522-1-yepeilin.cs@gmail.com>
References: <20200801020841.227522-1-yepeilin.cs@gmail.com>
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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@vger.kernel.org
Fixes: 201766a20e30 ("ptrace: add PTRACE_GET_SYSCALL_INFO request")
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
---
Change in v3:
    - Remove unnecessary `__aligned__` attribute. (Suggested by
      Dmitry V. Levin <ldv@altlinux.org>)

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

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