vim-patch:9.2.0738: ml_recover() may write beyond block buffer (#40446)

Problem:  A crafted swap file can cause an out-of-bounds write during
          recovery when the same block is referenced twice with
          different pe_page_count values (cipher-creator)
Solution: Check hp->bh_page_count against page_count after mf_get() and
          clamp page_count to the actual block size.

closes: vim/vim#20645

43939cf9eb

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/memline.c

@@ -1111,6 +1111,18 @@ void ml_recover(bool checkext)
           // Append all the lines in this block.
           bool has_error = false;
 
+          // Verify the cached block's actual size matches the
+          // pointer entry's pe_page_count.  mf_get() cache hits
+          // return the original block without resizing, so a
+          // crafted swap file referencing the same block twice
+          // with different pe_page_count values would cause an
+          // OOB write below.
+          if (hp->bh_page_count != page_count) {
+            error++;
+            ml_append(lnum++, _("??? BLOCK PAGE COUNT MISMATCH"), 0, true);
+            page_count = hp->bh_page_count;
+          }
+
           // Check the length of the block.
           // If wrong, use the length given in the pointer block.
           if (page_count * mfp->mf_page_size != dp->db_txt_end) {

test/old/testdir/test_recover.vim

@@ -519,8 +519,8 @@ func Test_recover_corrupted_swap_file1()
   call assert_match('???ILLEGAL BLOCK NUMBER', content)
   call delete(target)
   bw!
-"
-"  " Test 2: Segfault
+
+  " Test 2: Segfault
   new
   let sample = 'samples/recover-crash2.swp'
   let target = '.Xpoc2.swp'  " Xpoc1.swp (non-hidden) doesn't work in Nvim
@@ -536,6 +536,19 @@ func Test_recover_corrupted_swap_file1()
   call assert_match('???LINES MISSING', content)
   call delete(target)
   bw!
+
+  " Test 3: wrong page_count header
+  new
+  let sample = 'samples/recover-mismatch-pc.swp'
+  let target = 'Xmismatch-pc.swp'
+  call writefile(readblob(sample), target, 'bD')
+  try
+    sil noa recover! Xmismatch-pc.swp
+  catch
+  endtry
+  " Verifies no crash occurs. The OOB write is only reliably triggered
+  " interactively due to memory pressure evicting blocks in the test runner.
+  bw!
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab