[PATCH] soc: qcom: mdt_loader: fix off-by-one heap overflow in segment name

[Kenneth Kabogo <kennethkabogo2@gmail.com>]

mdt_load_split_segment() builds a segment firmware filename by
overwriting the last three characters of a kstrdup'd copy of fw_name
with "b%02d" formatted from segment. %02d is a minimum field width,
not a cap: for segment in [0, 99] this produces exactly "bNN" + NUL,
which fits the buffer, but for segment >= 100 it writes "b100" or
wider, one or more bytes past the end of the kstrdup allocation.

segment is not bounded to [0, 99] anywhere before this point. It is a
raw index into the firmware's ELF program header table, ultimately
bounded only by e_phnum, a 16-bit ELF field (max 65535). The existing
check in mdt_header_valid() only verifies that the program header
table fits inside the firmware file, not that e_phnum itself is sane,
so a small firmware file can supply e_phnum > 100 and reach this path
with segment >= 100 from either qcom_mdt_read_metadata() (the hash
segment scan) or the main loadable-segment loop in
qcom_mdt_load_no_init(), whenever the image uses the split-firmware
convention.

segment was never valid past 99 under this driver's own "bNN" naming
scheme, so reject it outright instead of widening the buffer.

Found by code inspection while auditing the Qualcomm PIL/PAS
firmware-authentication flow. No reproducer beyond code inspection is
available; I don't have affected hardware to trigger it on.

Signed-off-by: Kenneth Kabogo <kennethkabogo2@gmail.com>
---
 drivers/soc/qcom/mdt_loader.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/soc/qcom/mdt_loader.c b/drivers/soc/qcom/mdt_loader.c
index 002100fe2..0d55e9e0a 100644
--- a/drivers/soc/qcom/mdt_loader.c
+++ b/drivers/soc/qcom/mdt_loader.c
@@ -77,6 +77,9 @@ static ssize_t mdt_load_split_segment(void *ptr, const struct elf32_phdr *phdrs,
 	if (strlen(fw_name) < 4)
 		return -EINVAL;
 
+	if (segment > 99)
+		return -EINVAL;
+
 	char *seg_name __free(kfree) = kstrdup(fw_name, GFP_KERNEL);
 	if (!seg_name)
 		return -ENOMEM;
-- 
2.55.0