FAILED: patch "[PATCH] ubifs: Fix read out-of-bounds in ubifs_wbuf_write_nolock()" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 4f2262a334641e05f645364d5ade1f565c85f20b Mon Sep 17 00:00:00 2001
From: Zhihao Cheng <[email protected]>
Date: Mon, 27 Dec 2021 11:22:40 +0800
Subject: [PATCH] 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].

index 789a7813f3fa..1607a3c76681 100644
@@ -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;
 	}