[PATCH v3 1/2] platform/x86: hp-bioscfg: fix 16-byte heap overflow for empty auth token

hp_calculate_security_buffer() special-cases an empty authentication
string and returns a fixed 4 bytes (sizeof(u16) * 2). But
hp_populate_security_buffer() does not special-case that same input:
for any authentication string that does not start with BEAM_PREFIX,
including the empty string, it always builds "UTF_PREFIX +
authentication" and converts the result to UTF-16, writing a 2-byte
length header plus 2 bytes per character of "<utf-16/>" (9 characters),
20 bytes total, regardless of how long "authentication" itself is.

The caller, hp_set_attribute(), sizes its kmalloc() buffer using
hp_calculate_security_buffer()'s return value, so for an empty
authentication token it allocates 4 bytes for the security area but
hp_populate_security_buffer() then writes 20 bytes into it, causing a
16-byte heap buffer overflow.

The authentication token used here is the current admin/setup
password, which is an empty string by default until one is
configured. Any write to a writable BIOS attribute while no admin
password has been set reaches this path.

Fix by removing the special-case early return for an empty string in
hp_calculate_security_buffer(). The generic calculation that follows
already accounts for the UTF_PREFIX correctly, which naturally yields
the same 20 bytes that hp_populate_security_buffer() writes for an empty
string, avoiding duplicate logic for special cases.

Fixes: b2715aa2e135 ("platform/x86: hp-bioscfg: spmobj-attributes")
Reported-by: Josh Snyder <[email protected]>
Closes: https://lore.kernel.org/platform-driver-x86/[email protected]/
Cc: [email protected]
Signed-off-by: Muhammad Bilal <[email protected]>
---
Changes in v3:
  - Remove the special-case return entirely instead of adjusting its
    formula, avoiding code duplication as suggested by Ilpo Järvinen.
  - Credit Josh Snyder who previously noted this approach.

Changes in v2:
  - None for this patch; resubmitted as part of the v2 series.

Link: https://lore.kernel.org/r/[email protected] [v1]
Link: https://lore.kernel.org/r/[email protected] [v2]
---
 drivers/platform/x86/hp/hp-bioscfg/spmobj-attributes.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/drivers/platform/x86/hp/hp-bioscfg/spmobj-attributes.c b/drivers/platform/x86/hp/hp-bioscfg/spmobj-attributes.c
index 4d94e48c1a4c..136585141e6e 100644
--- a/drivers/platform/x86/hp/hp-bioscfg/spmobj-attributes.c
+++ b/drivers/platform/x86/hp/hp-bioscfg/spmobj-attributes.c
@@ -47,10 +47,6 @@ size_t hp_calculate_security_buffer(const char *authentication)
 	if (!authentication)
 		return sizeof(u16) * 2;
 
-	authlen = strlen(authentication);
-	if (!authlen)
-		return sizeof(u16) * 2;
-
 	authlen = strlen(authentication);
 	size = sizeof(u16) + authlen * sizeof(u16);
 	if (!strstarts(authentication, BEAM_PREFIX))
-- 
2.43.0