[PATCH] ubifs: fix out-of-bounds write in LPT commit padding

[Jenny Guanni Qu]

In write_cnodes(), when writing LPT data to a LEB, the code pads the
write buffer to min_io_size alignment using:

    memset(buf + offs, 0xff, alen - wlen)

where wlen = offs - from and alen = ALIGN(wlen, min_io_size). The
buffer (c->lpt_buf) is allocated with size c->leb_size. When offs is
near leb_size and from is non-zero, the alignment padding can push
the memset past the end of the buffer.

For example, with leb_size=4096, offs=4000, from=3584, min_io_size=1024:
  wlen=416, alen=1024, padding=608 bytes at offset 4000
  writes to offsets 4000..4607, 512 bytes past the 4096-byte buffer

Clamp the padding size to not exceed the buffer boundary in all 4
instances of this pattern.

The OOB write was confirmed with KASAN using a test module that
reproduces the arithmetic with matching buffer and alignment values.

Fixes: 1e51764a3c2a ("UBIFS: add new flash file system")
Reported-by: Klaudia Kloc <klaudia at vidocsecurity.com>
Reported-by: Dawid Moczadło <dawid at vidocsecurity.com>
Tested-by: Jenny Guanni Qu <qguanni at gmail.com>
Signed-off-by: Jenny Guanni Qu <qguanni at gmail.com>
---
 fs/ubifs/lpt_commit.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/ubifs/lpt_commit.c b/fs/ubifs/lpt_commit.c
index 07351fdce722..77a7a2cd418f 100644
--- a/fs/ubifs/lpt_commit.c
+++ b/fs/ubifs/lpt_commit.c
@@ -402,7 +402,7 @@ static int write_cnodes(struct ubifs_info *c)
 			wlen = offs - from;
 			if (wlen) {
 				alen = ALIGN(wlen, c->min_io_size);
-				memset(buf + offs, 0xff, alen - wlen);
+				memset(buf + offs, 0xff, min_t(int, alen - wlen, c->leb_size - offs));
 				err = ubifs_leb_write(c, lnum, buf + from, from,
 						       alen);
 				if (err)
@@ -461,7 +461,7 @@ static int write_cnodes(struct ubifs_info *c)
 		if (offs + c->lsave_sz > c->leb_size) {
 			wlen = offs - from;
 			alen = ALIGN(wlen, c->min_io_size);
-			memset(buf + offs, 0xff, alen - wlen);
+			memset(buf + offs, 0xff, min_t(int, alen - wlen, c->leb_size - offs));
 			err = ubifs_leb_write(c, lnum, buf + from, from, alen);
 			if (err)
 				return err;
@@ -487,7 +487,7 @@ static int write_cnodes(struct ubifs_info *c)
 		if (offs + c->ltab_sz > c->leb_size) {
 			wlen = offs - from;
 			alen = ALIGN(wlen, c->min_io_size);
-			memset(buf + offs, 0xff, alen - wlen);
+			memset(buf + offs, 0xff, min_t(int, alen - wlen, c->leb_size - offs));
 			err = ubifs_leb_write(c, lnum, buf + from, from, alen);
 			if (err)
 				return err;
@@ -510,7 +510,7 @@ static int write_cnodes(struct ubifs_info *c)
 	/* Write remaining data in buffer */
 	wlen = offs - from;
 	alen = ALIGN(wlen, c->min_io_size);
-	memset(buf + offs, 0xff, alen - wlen);
+	memset(buf + offs, 0xff, min_t(int, alen - wlen, c->leb_size - offs));
 	err = ubifs_leb_write(c, lnum, buf + from, from, alen);
 	if (err)
 		return err;
-- 
2.34.1