AZL-88556

In the Linux kernel, the following vulnerability has been resolved:

ext4: don't zero the entire extent if EXT4EXTDATAPARTIALVALID1

When allocating initialized blocks from a large unwritten extent, or when splitting an unwritten extent during end I/O and converting it to initialized, there is currently a potential issue of stale data if the extent needs to be split in the middle.

   0  A      B  N
   [UUUUUUUUUUUU]    U: unwritten extent
   [--DDDDDDDD--]    D: valid data
      |<-  ->| ----> this range needs to be initialized

ext4splitextent() first try to split this extent at B with EXT4EXTDATAENTIREVALID1 and EXT4EXTMAYZEROOUT flag set, but ext4splitextentat() failed to split this extent due to temporary lack of space. It zeroout B to N and mark the entire extent from 0 to N as written.

   0  A      B  N
   [WWWWWWWWWWWW]    W: written extent
   [SSDDDDDDDDZZ]    Z: zeroed, S: stale data

ext4splitextent() then try to split this extent at A with EXT4EXTDATA_VALID2 flag set. This time, it split successfully and left a stale written extent from 0 to A.

   0  A      B   N
   [WW|WWWWWWWWWW]
   [SS|DDDDDDDDZZ]

Fix this by pass EXT4EXTDATAPARTIALVALID1 to ext4splitextentat() when splitting at B, don't convert the entire extent to written and left it as unwritten after zeroing out B to N. The remaining work is just like the standard two-part split. ext4splitextent() will pass the EXT4EXTDATAVALID2 flag when it calls ext4splitextent_at() for the second time, allowing it to properly handle the split. If the split is successful, it will keep extent from 0 to A as unwritten.