[PATCH 5.15 114/120] ovl: Use "buf" flexible array for memcpy() destination

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From: Kees Cook <keescook@xxxxxxxxxxxx>

commit cf8aa9bf97cadf85745506c6a3e244b22c268d63 upstream.

The "buf" flexible array needs to be the memcpy() destination to avoid
false positive run-time warning from the recent FORTIFY_SOURCE
hardening:

  memcpy: detected field-spanning write (size 93) of single field "&fh->fb"
  at fs/overlayfs/export.c:799 (size 21)

Reported-by: syzbot+9d14351a171d0d1c7955@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/all/000000000000763a6c05e95a5985@xxxxxxxxxx/
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/overlayfs/export.c    |    2 +-
 fs/overlayfs/overlayfs.h |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/fs/overlayfs/export.c
+++ b/fs/overlayfs/export.c
@@ -791,7 +791,7 @@ static struct ovl_fh *ovl_fid_to_fh(stru
 		return ERR_PTR(-ENOMEM);
 
 	/* Copy unaligned inner fh into aligned buffer */
-	memcpy(&fh->fb, fid, buflen - OVL_FH_WIRE_OFFSET);
+	memcpy(fh->buf, fid, buflen - OVL_FH_WIRE_OFFSET);
 	return fh;
 }
 
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -107,7 +107,7 @@ struct ovl_fh {
 	u8 padding[3];	/* make sure fb.fid is 32bit aligned */
 	union {
 		struct ovl_fb fb;
-		u8 buf[0];
+		DECLARE_FLEX_ARRAY(u8, buf);
 	};
 } __packed;
 





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