FAILED: patch "[PATCH] kprobe: Do not use uaccess functions to access kernel memory" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 2c4f1fcbef0bc324830bc2fb1a264c08ec93dec5 Mon Sep 17 00:00:00 2001
From: Changbin Du <[email protected]>
Date: Fri, 25 Jan 2019 23:10:50 +0800
Subject: [PATCH] kprobe: Do not use uaccess functions to access kernel memory
that can fault

The userspace can ask kprobe to intercept strings at any memory address, including invalid kernel address. In this case, fetch_store_strlen() would crash since it uses general usercopy function, and user access functions are no longer allowed to access kernel memory.

For example, we can crash the kernel by doing something as below:

$ sudo kprobe 'p:do_sys_open +0(+0(%si)):string'

[  103.620391] BUG: GPF in non-whitelisted uaccess (non-canonical address?)
[  103.622104] general protection fault: 0000 [#1] SMP PTI
[  103.623424] CPU: 10 PID: 1046 Comm: cat Not tainted 5.0.0-rc3-00130-gd73aba1-dirty #96
[  103.625321] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-2-g628b2e6-dirty-20190104_103505-linux 04/01/2014
[  103.628284] RIP: 0010:process_fetch_insn+0x1ab/0x4b0
[  103.629518] Code: 10 83 80 28 2e 00 00 01 31 d2 31 ff 48 8b 74 24 28 eb 0c 81 fa ff 0f 00 00 7f 1c 85 c0 75 18 66 66 90 0f ae e8 48 63
 ca 89 f8 <8a> 0c 31 66 66 90 83 c2 01 84 c9 75 dc 89 54 24 34 89 44 24 28 48
[  103.634032] RSP: 0018:ffff88845eb37ce0 EFLAGS: 00010246
[  103.635312] RAX: 0000000000000000 RBX: ffff888456c4e5a8 RCX: 0000000000000000
[  103.637057] RDX: 0000000000000000 RSI: 2e646c2f6374652f RDI: 0000000000000000
[  103.638795] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
[  103.640556] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
[  103.642297] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[  103.644040] FS:  0000000000000000(0000) GS:ffff88846f000000(0000) knlGS:0000000000000000
[  103.646019] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  103.647436] CR2: 00007ffc79758038 CR3: 0000000463360006 CR4: 0000000000020ee0
[  103.649147] Call Trace:
[  103.649781]  ? sched_clock_cpu+0xc/0xa0
[  103.650747]  ? do_sys_open+0x5/0x220
[  103.651635]  kprobe_trace_func+0x303/0x380
[  103.652645]  ? do_sys_open+0x5/0x220
[  103.653528]  kprobe_dispatcher+0x45/0x50
[  103.654682]  ? do_sys_open+0x1/0x220
[  103.655875]  kprobe_ftrace_handler+0x90/0xf0
[  103.657282]  ftrace_ops_assist_func+0x54/0xf0
[  103.658564]  ? __call_rcu+0x1dc/0x280
[  103.659482]  0xffffffffc00000bf
[  103.660384]  ? __ia32_sys_open+0x20/0x20
[  103.661682]  ? do_sys_open+0x1/0x220
[  103.662863]  do_sys_open+0x5/0x220
[  103.663988]  do_syscall_64+0x60/0x210
[  103.665201]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  103.666862] RIP: 0033:0x7fc22fadccdd
[  103.668034] Code: 48 89 54 24 e0 41 83 e2 40 75 32 89 f0 25 00 00 41 00 3d 00 00 41 00 74 24 89 f2 b8 01 01 00 00 48 89 fe bf 9c ff ff
 ff 0f 05 <48> 3d 00 f0 ff ff 77 33 f3 c3 66 0f 1f 84 00 00 00 00 00 48 8d 44
[  103.674029] RSP: 002b:00007ffc7972c3a8 EFLAGS: 00000287 ORIG_RAX: 0000000000000101
[  103.676512] RAX: ffffffffffffffda RBX: 0000562f86147a21 RCX: 00007fc22fadccdd
[  103.678853] RDX: 0000000000080000 RSI: 00007fc22fae1428 RDI: 00000000ffffff9c
[  103.681151] RBP: ffffffffffffffff R08: 0000000000000000 R09: 0000000000000000
[  103.683489] R10: 0000000000000000 R11: 0000000000000287 R12: 00007fc22fce90a8
[  103.685774] R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000000
[  103.688056] Modules linked in:
[  103.689131] ---[ end trace 43792035c28984a1 ]---

This can be fixed by using probe_mem_read() instead, as it can handle faulting kernel memory addresses, which kprobes can legitimately do.

index d5fb09ebba8b..9eaf07f99212 100644
@@ -861,22 +861,14 @@ static const struct file_operations kprobe_profile_ops = {
 static nokprobe_inline int
 fetch_store_strlen(unsigned long addr)
 {
-	mm_segment_t old_fs;
 	int ret, len = 0;
 	u8 c;
 
-	old_fs = get_fs();
-	set_fs(KERNEL_DS);
-	pagefault_disable();
-
 	do {
-		ret = __copy_from_user_inatomic(&c, (u8 *)addr + len, 1);
+		ret = probe_mem_read(&c, (u8 *)addr + len, 1);
 		len++;
 	} while (c && ret == 0 && len < MAX_STRING_SIZE);
 
-	pagefault_enable();
-	set_fs(old_fs);
-
 	return (ret < 0) ? ret : len;
 }