[PATCH] platform/x86: hp-bioscfg: fix slab-out-of-bounds write in hp_convert_hexstr_to_str

hp_convert_hexstr_to_str() sizes its output buffer using the raw
hex-encoded input length, but the decoded string written into it can
need up to two bytes of output per five bytes of input when escaping
'\\', '\r', '\n', or '\t', plus one more byte for the NUL terminator
written unconditionally after the decode loop. For a short encoded
value the decoded length plus terminator can reach or exceed the
allocated size, causing an out-of-bounds slab write.

KASAN caught a one-byte overflow during BIOS attribute enumeration
on boot, triggered by a one-byte encoded input value:

BUG: KASAN: slab-out-of-bounds in hp_convert_hexstr_to_str+0x6d8/0x710 [hp_bioscfg]
Write of size 1 at addr ffff8881032e5d81 by task (udev-worker)/520
The buggy address is located 0 bytes to the right of
allocated 1-byte region [ffff8881032e5d80, ffff8881032e5d81)

Size the allocation to the worst-case decoded length, two bytes per
five-byte input chunk, plus the terminator, instead of the raw input
length.

Fixes: a34fc329b189 ("platform/x86: hp-bioscfg: bioscfg")
Cc: [email protected]
Signed-off-by: Muhammad Bilal <[email protected]>
---
 drivers/platform/x86/hp/hp-bioscfg/bioscfg.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/platform/x86/hp/hp-bioscfg/bioscfg.c b/drivers/platform/x86/hp/hp-bioscfg/bioscfg.c
--- a/drivers/platform/x86/hp/hp-bioscfg/bioscfg.c
+++ b/drivers/platform/x86/hp/hp-bioscfg/bioscfg.c
@@ -442,7 +442,7 @@ int hp_convert_hexstr_to_str(const char *input, u32 input_len, char **str, int *len)
 	*len = 0;
 	*str = NULL;
 
-	new_str = kmalloc(input_len, GFP_KERNEL);
+	new_str = kmalloc(2 * DIV_ROUND_UP(input_len, 5) + 1, GFP_KERNEL);
 	if (!new_str)
 		return -ENOMEM;