[PATCH] lib: Always NUL terminate ucs2_as_utf8

[Chris Wilson]

If the caller, in this case efivarfs_callback(), only provides sufficent
room for the expanded utf8 and not enough to include the terminating NUL
byte, that NUL byte is skipped. When the caller then interprets it as a
string, it may then read from past its allocated memory:

[  170.605647] WARNING: kmemcheck: Caught 8-bit read from uninitialized memory (ffff8804079ae786)
[  170.605677] 436f6e4f757400004c44322d35363062663538612d316530642d346437652d39
[  170.606037]  i i i i i i u u u u u u u u u u u u u u u u u u u u u u u u u u
[  170.606236]              ^
[  170.606243] RIP: 0010:[<ffffffff813a251f>]  [<ffffffff813a251f>] efivar_variable_is_removable+0xaf/0xf0
[  170.606346] RSP: 0018:ffff880408e73c20  EFLAGS: 00010206
[  170.606352] RAX: 0000000000000000 RBX: 0000000000000006 RCX: 0000000000000006
[  170.606359] RDX: 0000000000000000 RSI: 0000000000000074 RDI: ffff880408e73c30
[  170.606365] RBP: ffff880408e73c80 R08: 0000000000000006 R09: 000000000000008c
[  170.606371] R10: 0000000000000006 R11: 0000000000000000 R12: ffffffff8166ed20
[  170.606378] R13: 11d293ca8be4df61 R14: ffffffff81773834 R15: ffff8804079ae780
[  170.606385] FS:  0000000000000000(0000) GS:ffff88041ca00000(0000) knlGS:0000000000000000
[  170.606392] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  170.606399] CR2: ffff880409cbe4c0 CR3: 00000004085fd000 CR4: 00000000001406f0
[  170.606405]  [<ffffffff811eb938>] efivarfs_callback+0xf8/0x275
[  170.606418]  [<ffffffff813a3368>] efivar_init+0x248/0x2e0
[  170.606440]  [<ffffffff811eb6b4>] efivarfs_fill_super+0xb4/0xf0
[  170.606452]  [<ffffffff811333e7>] mount_single+0x87/0xb0
[  170.606463]  [<ffffffff811eb5f3>] efivarfs_mount+0x13/0x20
[  170.606475]  [<ffffffff81133480>] mount_fs+0x10/0x90
[  170.606497]  [<ffffffff8114c732>] vfs_kern_mount+0x62/0x100
[  170.606508]  [<ffffffff8114ecb0>] do_mount+0x1e0/0xcd0
[  170.606519]  [<ffffffff8114fa9f>] SyS_mount+0x8f/0xd0
[  170.606530]  [<ffffffff81451d1f>] entry_SYSCALL_64_fastpath+0x17/0x93
[  170.606542]  [<ffffffffffffffff>] 0xffffffffffffffff

Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Cc: Jason Andryuk <jandryuk@xxxxxxxxx>
Cc: Matthew Garrett <mjg59@xxxxxxxxxx>
Cc: Laszlo Ersek <lersek@xxxxxxxxxx>
Cc: Peter Jones <pjones@xxxxxxxxxx>
Cc: linux-efi@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 drivers/firmware/efi/vars.c |  2 +-
 lib/ucs2_string.c           | 15 ++++++++++-----
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c
index 0ac594c0a234..8dd503bac35d 100644
--- a/drivers/firmware/efi/vars.c
+++ b/drivers/firmware/efi/vars.c
@@ -166,7 +166,7 @@ validate_ascii_string(efi_char16_t *var_name, int match, u8 *buffer,
 
 struct variable_validate {
 	efi_guid_t vendor;
-	char *name;
+	const char *name;
 	bool (*validate)(efi_char16_t *var_name, int match, u8 *data,
 			 unsigned long len);
 };
diff --git a/lib/ucs2_string.c b/lib/ucs2_string.c
index f0b323abb4c6..fb8d03966656 100644
--- a/lib/ucs2_string.c
+++ b/lib/ucs2_string.c
@@ -85,29 +85,34 @@ ucs2_as_utf8(u8 *dest, const ucs2_char_t *src, unsigned long maxlength)
 	unsigned long j = 0;
 	unsigned long limit = ucs2_strnlen(src, maxlength);
 
-	for (i = 0; maxlength && i < limit; i++) {
+	if (maxlength == 0)
+		return 0;
+
+	for (i = 0; i < limit; i++) {
 		u16 c = src[i];
 
 		if (c >= 0x800) {
-			if (maxlength < 3)
+			if (maxlength <= 3)
 				break;
 			maxlength -= 3;
 			dest[j++] = 0xe0 | (c & 0xf000) >> 12;
 			dest[j++] = 0x80 | (c & 0x0fc0) >> 6;
 			dest[j++] = 0x80 | (c & 0x003f);
 		} else if (c >= 0x80) {
-			if (maxlength < 2)
+			if (maxlength <= 2)
 				break;
 			maxlength -= 2;
 			dest[j++] = 0xc0 | (c & 0x7c0) >> 6;
 			dest[j++] = 0x80 | (c & 0x03f);
 		} else {
+			if (maxlength <= 1)
+				break;
 			maxlength -= 1;
 			dest[j++] = c & 0x7f;
 		}
 	}
-	if (maxlength)
-		dest[j] = '\0';
+	dest[j] = '\0';
+
 	return j;
 }
 EXPORT_SYMBOL(ucs2_as_utf8);
-- 
2.8.1