[PATCH v2] ntdll: Fix read buffer overflow in "find_file_in_dir"

[Joachim Priesner]

If the VFAT_IOCTL_READDIR_BOTH logic in find_file_in_dir does not yield
any result, the NUL terminator of "unix_name" must be restored. Else
the following opendir() call will read past the end of "unix_name".

This currently causes CreateFile(... CREATE_NEW ...) for non-existing
files on FAT32 drives to fail.

Signed-off-by: Joachim Priesner 
---
v2: Simplified and added test program to increase chances of approval
(v1 title was: ntdll: Fix CreateFile for non-existing files on FAT file systems)

Test: The following test program will currently succeed when given a
non-existing path on a non-FAT32 drive and fail on a FAT32 drive.
With this patch, it succeeds on both FAT32 and non-FAT32 drives.

#include <windows.h>
  int main(int argc, char** argv) {
    return SUCCEEDED(CreateFile(argv[1], GENERIC_WRITE, 0, NULL, CREATE_NEW, FILE_ATTRIBUTE_NORMAL, NULL)) ? 0 : 1;
  }

---
 dlls/ntdll/unix/file.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/dlls/ntdll/unix/file.c b/dlls/ntdll/unix/file.c
index afb552be098..d12a3ffb119 100644
--- a/dlls/ntdll/unix/file.c
+++ b/dlls/ntdll/unix/file.c
@@ -2550,6 +2550,8 @@ static NTSTATUS find_file_in_dir( char *unix_name, int pos, const WCHAR *name, i
                         goto not_found;
                     }
                 }
+                /* if that did not work, restore previous state of unix_name */
+                unix_name[pos - 1] = 0;
             }
             close( fd );
         }
--
2.28.0