ubifs: Fix read out-of-bounds in ubifs_wbuf_write_nolock()

Function ubifs_wbuf_write_nolock() may access buf out of bounds in
following process:

ubifs_wbuf_write_nolock():
  aligned_len = ALIGN(len, 8);   // Assume len = 4089, aligned_len = 4096
  if (aligned_len <= wbuf->avail) ... // Not satisfy
  if (wbuf->used) {
    ubifs_leb_write()  // Fill some data in avail wbuf
    len -= wbuf->avail;   // len is still not 8-bytes aligned
    aligned_len -= wbuf->avail;
  }
  n = aligned_len >> c->max_write_shift;
  if (n) {
    n <<= c->max_write_shift;
    err = ubifs_leb_write(c, wbuf->lnum, buf + written,
                          wbuf->offs, n);
    // n > len, read out of bounds less than 8(n-len) bytes
  }

, which can be catched by KASAN:
  =========================================================
  BUG: KASAN: slab-out-of-bounds in ecc_sw_hamming_calculate+0x1dc/0x7d0
  Read of size 4 at addr ffff888105594ff8 by task kworker/u8:4/128
  Workqueue: writeback wb_workfn (flush-ubifs_0_0)
  Call Trace:
    kasan_report.cold+0x81/0x165
    nand_write_page_swecc+0xa9/0x160
    ubifs_leb_write+0xf2/0x1b0 [ubifs]
    ubifs_wbuf_write_nolock+0x421/0x12c0 [ubifs]
    write_head+0xdc/0x1c0 [ubifs]
    ubifs_jnl_write_inode+0x627/0x960 [ubifs]
    wb_workfn+0x8af/0xb80

Function ubifs_wbuf_write_nolock() accepts that parameter 'len' is not 8
bytes aligned, the 'len' represents the true length of buf (which is
allocated in 'ubifs_jnl_xxx', eg. ubifs_jnl_write_inode), so
ubifs_wbuf_write_nolock() must handle the length read from 'buf' carefully
to write leb safely.

Fetch a reproducer in [Link].

Fixes: 1e51764 ("UBIFS: add new flash file system")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=214785
Reported-by: Chengsong Ke <[email protected]>
Signed-off-by: Zhihao Cheng <[email protected]>
Signed-off-by: Richard Weinberger <[email protected]>

fs/ubifs/io.c

@@ -854,16 +854,42 @@ int ubifs_wbuf_write_nolock(struct ubifs_wbuf *wbuf, void *buf, int len)
 	 */
 	n = aligned_len >> c->max_write_shift;
 	if (n) {
-		n <<= c->max_write_shift;
+		int m = n - 1;
+
 		dbg_io("write %d bytes to LEB %d:%d", n, wbuf->lnum,
 		       wbuf->offs);
-		err = ubifs_leb_write(c, wbuf->lnum, buf + written,
-				      wbuf->offs, n);
+
+		if (m) {
+			/* '(n-1)<<c->max_write_shift < len' is always true. */
+			m <<= c->max_write_shift;
+			err = ubifs_leb_write(c, wbuf->lnum, buf + written,
+					      wbuf->offs, m);
+			if (err)
+				goto out;
+			wbuf->offs += m;
+			aligned_len -= m;
+			len -= m;
+			written += m;
+		}
+
+		/*
+		 * The non-written len of buf may be less than 'n' because
+		 * parameter 'len' is not 8 bytes aligned, so here we read
+		 * min(len, n) bytes from buf.
+		 */
+		n = 1 << c->max_write_shift;
+		memcpy(wbuf->buf, buf + written, min(len, n));
+		if (n > len) {
+			ubifs_assert(c, n - len < 8);
+			ubifs_pad(c, wbuf->buf + len, n - len);
+		}
+
+		err = ubifs_leb_write(c, wbuf->lnum, wbuf->buf, wbuf->offs, n);
 		if (err)
 			goto out;
 		wbuf->offs += n;
 		aligned_len -= n;
-		len -= n;
+		len -= min(len, n);
 		written += n;
 	}