On 04/20/16 10:37, Chris Wilson wrote:
> 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.
How does that occur? In efivarfs_callback() [fs/efivarfs/super.c], we have
len = ucs2_utf8size(entry->var.VariableName);
/* name, plus '-', plus GUID, plus NUL*/
name = kmalloc(len + 1 + EFI_VARIABLE_GUID_LEN + 1, GFP_KERNEL);
if (!name)
goto fail;
ucs2_as_utf8(name, entry->var.VariableName, len);
Instead, I think the following might be happening (note that RIP points
into efivar_variable_is_removable(), and I guess variable_matches()
(which is static) is inlined):
efivarfs_callback() [fs/efivarfs/super.c]
efivar_variable_is_removable() [drivers/firmware/efi/vars.c]
variable_matches() [drivers/firmware/efi/vars.c]
The bug seems to be in variable_matches(), which doesn't consider the
"len" parameter early enough. Namely, consider that we have the
following input:
- var_name: "a"
- len: 1
- match_name "ab"
In the first iteration of the loop (i.e., *match == 0):
- c = 'a'
- u = 'a'
- *match gets incremented to 1.
In the second iteration of the loop (i.e., *match == 1):
- c = 'b'
- u = <indeterminate value> (that is, undefined behavior),
because (*match == len).
This seems to be consistent with the error message "Caught 8-bit read
from uninitialized memory": namely, the array allocated for "name" in
efivarfs_callback() is indeed not pre-zeroed, and the ucs2_as_utf8()
function does not populate name[len] -- correctly, I would say.
So, I think the function that needs a fix is variable_matches().
(I don't disagree though that it could be useful to audit all
ucs2_as_utf8() calls.)
Thanks
Laszlo
> 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);
>
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