[PATCH v4 1/2] tools/bootconfig: Fix integer overflow and truncation in size checks

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Sashiko reported that on 32-bit systems, if an attacker crafts size in
the bootconfig footer such that adding BOOTCONFIG_FOOTER_SIZE wraps around
(for instance, if size is 0xFFFFFFFF), the size check in
load_xbc_from_initrd() can be bypassed:

    if (stat.st_size < size + BOOTCONFIG_FOOTER_SIZE) {
        pr_err("bootconfig size is too big\n");
        return -E2BIG;
    }

Furthermore, on 64-bit systems with an initrd > 4.29 GB, comparing a
corrupted 32-bit size (e.g. 0xFFFFFFFF) against
stat.st_size - BOOTCONFIG_FOOTER_SIZE can also bypass the check if
size is not bounded. Similarly, load_xbc_file() passes 64-bit stat.st_size
directly into the 32-bit int size parameter of load_xbc_fd(), truncating
large standalone files (>= 2GB).

In both cases, passing 0xFFFFFFFF to load_xbc_fd() truncates to -1,
resulting in malloc(0), an integer overflow in read(), and an
out-of-bounds null-byte write.

Fix this by:
1. Rejecting size > XBC_DATA_MAX or
   size > stat.st_size - BOOTCONFIG_FOOTER_SIZE in load_xbc_from_initrd().
2. Rejecting stat.st_size > XBC_DATA_MAX in load_xbc_file() before passing
   it to load_xbc_fd().
3. Checking size < 0 || size > XBC_DATA_MAX defensively in load_xbc_fd().

Fixes: 950313ebf79c ("tools: bootconfig: Add bootconfig command")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/20260909161113.16C691F00A3A@xxxxxxxxxxxxxxx/
Closes: https://lore.kernel.org/all/20260910010137.EE0431F000FF@xxxxxxxxxxxxxxx/
Assisted-by: Antigravity:gemini-3.8-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
Changes in v3:
 - Reject size > XBC_DATA_MAX and
   size > stat.st_size - BOOTCONFIG_FOOTER_SIZE in load_xbc_from_initrd()
   to prevent bypass on initrd > 4.29 GB.
 - Check stat.st_size > XBC_DATA_MAX in load_xbc_file() to prevent
   truncation on large standalone files.
 - Check size < 0 || size > XBC_DATA_MAX defensively in load_xbc_fd().
Changes in v2:
  - Add Cc: stable.
  - Also reject if "size > XBC_DATA_MAX", that is obviously wrong.
---
 tools/bootconfig/main.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/bootconfig/main.c b/tools/bootconfig/main.c
index 7dc9fff9b637..17d971d47f87 100644
--- a/tools/bootconfig/main.c
+++ b/tools/bootconfig/main.c
@@ -140,6 +140,9 @@ static int load_xbc_fd(int fd, char **buf, int size)
 {
 	int ret;
 
+	if (size < 0 || size > XBC_DATA_MAX)
+		return -EINVAL;
+
 	*buf = malloc(size + 1);
 	if (!*buf)
 		return -ENOMEM;
@@ -168,6 +171,13 @@ static int load_xbc_file(const char *path, char **buf)
 		return ret;
 	}
 
+	if (stat.st_size > XBC_DATA_MAX) {
+		pr_err("%s size is too big\n", path);
+		ret = -E2BIG;
+		close(fd);
+		return ret;
+	}
+
 	ret = load_xbc_fd(fd, buf, stat.st_size);
 
 	close(fd);
@@ -218,7 +228,8 @@ static int load_xbc_from_initrd(int fd, char **buf)
 	csum = le32toh(csum);
 
 	/* Wrong size error  */
-	if (stat.st_size < size + BOOTCONFIG_FOOTER_SIZE) {
+	if (size > XBC_DATA_MAX ||
+	    size > stat.st_size - BOOTCONFIG_FOOTER_SIZE) {
 		pr_err("bootconfig size is too big\n");
 		return -E2BIG;
 	}