bootconfig: Fix integer overflow problems

From: Masami Hiramatsu (Google) <[email protected]>

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 [email protected]

Reported-by Sashiko <[email protected]>

Closes https://lore.kernel.org/all/[email protected]/ open here

Closes https://lore.kernel.org/all/[email protected]/ open here

Assisted-by Antigravity:gemini-3.8-flash

Signed-off-by Masami Hiramatsu (Google) <[email protected]>

---
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(-)

tools/bootconfig/main.c +10 -0

index 7dc9fff9b637..17d971d47f87 100644
@@ -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);

From: Masami Hiramatsu (Google) <[email protected]>

Sashiko reported that in get_boot_config_from_initrd(), a crafted initrd with a huge bootconfig size (such as 0xFFFFFFFF) can cause the pointer arithmetic:

    data = ((void *)hdr) - size;

to wrap around on 32-bit systems (or when pointer subtraction overflows). Because data wraps around, the subsequent bounds check:

if ((unsigned long)data < initrd_start)

evaluates to false, bypassing the check. The kernel then calls xbc_calc_checksum(data, size), which attempts to read 4GB of memory, hitting unmapped pages and triggering a fatal kernel page fault during early boot. Furthermore, on 64-bit systems with an initrd > 4.29 GB, an unbounded 32-bit size can similarly bypass the initrd_start check.

Fix this by:
1. Ensuring the initrd is at least large enough to contain the bootconfig footer and verifying hdr is within the initrd bounds.
2. Checking that size does not exceed XBC_DATA_MAX and does not exceed the available space between initrd_start and hdr before performing pointer subtraction.

Fixes de462e5f1071 ("bootconfig: Fix to remove bootconfig data from initrd while boot")

Cc [email protected]

Reported-by Sashiko <[email protected]>

Closes https://lore.kernel.org/all/[email protected]/ open here

Assisted-by Antigravity:gemini-3.8-flash

Signed-off-by Masami Hiramatsu (Google) <[email protected]>

---
Changes in v4:
- Accurate the error message for size > XBC_DATA_MAX in get_boot_config_from_initrd().
- Remove redundant (and wrong) size >= XBC_DATA_MAX check from setup_boot_config().

---
 init/main.c |   25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

init/main.c +0 -0

index 2613d3f9b3ce..16749bb7a219 100644

init/main.c +13 -3

        data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;

@@ -294,16 +295,26 @@ static void * __init get_boot_config_from_initrd(size_t *_size)
 
 found:
        hdr = (u8 *)(data - 8);
+       if ((unsigned long)hdr < initrd_start)
+               return NULL;
+
        size = get_unaligned_le32(hdr);
        csum = get_unaligned_le32(hdr + 4);
 
-       data = ((void *)hdr) - size;
-       if ((unsigned long)data < initrd_start) {
-               pr_err("bootconfig size %d is greater than initrd size %ld\n",
+       if (size > XBC_DATA_MAX) {
+               pr_err("bootconfig size %u is greater than max size %d\n",
+                       size, XBC_DATA_MAX);
+               return NULL;
+       }
+
+       if (size > ((unsigned long)hdr - initrd_start)) {
+               pr_err("bootconfig size %u is greater than initrd size %lu\n",
                        size, initrd_end - initrd_start);
                return NULL;
        }
 
+       data = ((void *)hdr) - size;
+
        if (xbc_calc_checksum(data, size) != csum) {
                pr_err("bootconfig checksum failed\n");
                return NULL;

Quoted 51 lines · Masami Hiramatsu (Google)

From: Masami Hiramatsu (Google) <[email protected]>

Sashiko reported that in get_boot_config_from_initrd(), a crafted initrd with a huge bootconfig size (such as 0xFFFFFFFF) can cause the pointer arithmetic:

    data = ((void *)hdr) - size;

to wrap around on 32-bit systems (or when pointer subtraction overflows). Because data wraps around, the subsequent bounds check:

if ((unsigned long)data < initrd_start)

evaluates to false, bypassing the check. The kernel then calls xbc_calc_checksum(data, size), which attempts to read 4GB of memory, hitting unmapped pages and triggering a fatal kernel page fault during early boot. Furthermore, on 64-bit systems with an initrd > 4.29 GB, an unbounded 32-bit size can similarly bypass the initrd_start check.

Fix this by:
1. Ensuring the initrd is at least large enough to contain the bootconfig footer and verifying hdr is within the initrd bounds.
2. Checking that size does not exceed XBC_DATA_MAX and does not exceed the available space between initrd_start and hdr before performing pointer subtraction.

Fixes de462e5f1071 ("bootconfig: Fix to remove bootconfig data from initrd while boot")

Cc [email protected]

Reported-by Sashiko <[email protected]>

Closes https://lore.kernel.org/all/[email protected]/ open here

Assisted-by Antigravity:gemini-3.8-flash

Signed-off-by Masami Hiramatsu (Google) <[email protected]>

---
Changes in v4:
- Accurate the error message for size > XBC_DATA_MAX in get_boot_config_from_initrd().
- Remove redundant (and wrong) size >= XBC_DATA_MAX check from setup_boot_config().

---
 init/main.c |   25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

init/main.c +0 -0

index 2613d3f9b3ce..16749bb7a219 100644
@@ -277,7 +277,8 @@ static void * __init get_boot_config_from_initrd(size_t *_size)
        u8 *hdr;
        int i;
 

init/main.c static void * __init get_boot_config_from_initrd(size_t *_size) · line 282

Quoted 4 lines

init/main.c +2 -1

-       if (!initrd_end)
+       if (!initrd_end || initrd_end < initrd_start ||
+           initrd_end - initrd_start < BOOTCONFIG_MAGIC_LEN + 8)
                return NULL;

(nit) how about moving BOOTCONFIG_FOOTER_SIZE from tools and using it instead?

Quoted 33 lines

init/main.c +13 -3

        data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;

@@ -294,16 +295,26 @@ static void * __init get_boot_config_from_initrd(size_t *_size)
 
 found:
        hdr = (u8 *)(data - 8);
+       if ((unsigned long)hdr < initrd_start)
+               return NULL;
+
        size = get_unaligned_le32(hdr);
        csum = get_unaligned_le32(hdr + 4);
 
-       data = ((void *)hdr) - size;
-       if ((unsigned long)data < initrd_start) {
-               pr_err("bootconfig size %d is greater than initrd size %ld\n",
+       if (size > XBC_DATA_MAX) {
+               pr_err("bootconfig size %u is greater than max size %d\n",
+                       size, XBC_DATA_MAX);
+               return NULL;
+       }
+
+       if (size > ((unsigned long)hdr - initrd_start)) {
+               pr_err("bootconfig size %u is greater than initrd size %lu\n",
                        size, initrd_end - initrd_start);
                return NULL;
        }
 
+       data = ((void *)hdr) - size;
+
        if (xbc_calc_checksum(data, size) != csum) {
                pr_err("bootconfig checksum failed\n");
                return NULL;

init/main.c static void __init setup_boot_config(void) · line 402

Quoted 9 lines

init/main.c +0 -6

@@ -394,12 +405,6 @@ static void __init setup_boot_config(void)
                return;
        }
 
-       if (size >= XBC_DATA_MAX) {
-               pr_err("bootconfig size %ld greater than max size %d\n",
-                       (long)size, XBC_DATA_MAX);
-               return;
-       }
-

Nice cleanup!

init/main.c static void __init setup_boot_config(void) · line 410

Quoted 4 lines

init/main.c +0 -0

        ret = xbc_init(data, size, &msg, &pos);
        if (ret < 0) {
                if (pos < 0)

Thanks for doing this work!

Reviewed-by Sang-Heon Jeon <[email protected]>