[PATCH] jffs2: validate summary offset in the XIP scan path

[hyunjungg <hj351016@gmail.com>]

From: Hyunjung Ko <hj351016@gmail.com>

jffs2_scan_eraseblock() locates the summary node at the end of an
eraseblock and derives both a pointer and a length from the on-flash
summary marker:

	sumptr = buf + je32_to_cpu(sm->offset);
	sumlen = c->sector_size - je32_to_cpu(sm->offset);

In the non-XIP branch this is followed by

	if (sumlen > c->sector_size)
		goto full_scan;

so a bogus sm->offset (for which sector_size - offset underflows to a
large value) is rejected.  The XIP branch (taken when the whole device
was successfully mtd_point()ed, e.g. an mtdram-backed MTD) is missing
that check.

On a crafted image an attacker controls the bytes of the summary marker
at the end of the last eraseblock.  Setting sm->magic == JFFS2_SUM_MAGIC
and sm->offset > sector_size makes sumlen wrap to ~4 GiB while sumptr
still points inside the mapped flash.  jffs2_sum_scan_sumnode() then runs

	crc = crc32(0, summary->sum,
		    sumsize - sizeof(struct jffs2_raw_summary));

with the attacker-controlled sumsize, reading far past the end of the
mtd_point() mapping -- an out-of-bounds read (KASAN: vmalloc OOB /
general protection fault) of controlled length.  The preceding header
CRC checks are computed over data read from the attacker-controlled
flash and can be satisfied, so they do not prevent the over-read.

Mirror the non-XIP guard in the XIP branch.

Reproduced under KASAN on 7.4-rc1 by mounting a crafted mtdram-backed
image (the XIP mtd_point() branch): KASAN reports a vmalloc-out-of-bounds
read in the summary-node crc32 over the mapped flash.

Signed-off-by: Hyunjung Ko <hj351016@gmail.com>
---
 fs/jffs2/scan.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/jffs2/scan.c b/fs/jffs2/scan.c
--- a/fs/jffs2/scan.c
+++ b/fs/jffs2/scan.c
@@ -495,6 +495,10 @@ static int jffs2_scan_eraseblock (struct jffs2_sb_info *c, struct jffs2_eraseblo
 			if (je32_to_cpu(sm->magic) == JFFS2_SUM_MAGIC) {
 				sumptr = buf + je32_to_cpu(sm->offset);
 				sumlen = c->sector_size - je32_to_cpu(sm->offset);
+
+				/* sm->offset may be garbage even if MAGIC matched */
+				if (sumlen > c->sector_size)
+					goto full_scan;
 			}
 		} else {
 			/* If NAND flash, read a whole page of it. Else just the end */