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]/
Assisted-by: Antigravity:gemini-3.8-flash
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>
---
init/main.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)
diff --git a/init/main.c b/init/main.c
index 2613d3f9b3ce..60c27d5f2bce 100644
--- a/init/main.c
+++ b/init/main.c
@@ -277,7 +277,8 @@ static void * __init get_boot_config_from_initrd(size_t
*_size)
u8 *hdr;
int i;
- if (!initrd_end)
+ if (!initrd_end || initrd_end < initrd_start ||
+ initrd_end - initrd_start < BOOTCONFIG_MAGIC_LEN + 8)
return NULL;
data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;
@@ -294,16 +295,21 @@ 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 ||
+ 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;
