[PATCH v2] xfs: assure zeroed memory buffers for certain kmem allocations

[Bill O'Donnell <billodo@redhat.com>]

Guarantee zeroed memory buffers for cases where potential memory
leak to disk can occur. In these cases, kmem_alloc is used and
doesn't zero the buffer, opening the possibility of information
leakage to disk.

Introduce a xfs_buf_flag, XBF_ZERO, to indicate a request for a zeroed
buffer, and use existing infrastucture (xfs_buf_allocate_memory) to
obtain the already zeroed buffer from kernel memory.

This solution avoids the performance issue that would occur if a
wholesale change to replace kmem_alloc with kmem_zalloc was done.

Signed-off-by: Bill O'Donnell <billodo@redhat.com>
---
v2: zeroed buffer not required for XBF_READ case. Correct placement
    and rename the XBF_ZERO flag.

 fs/xfs/xfs_buf.c       | 9 +++++++--
 fs/xfs/xfs_buf.h       | 2 ++
 fs/xfs/xfs_trans_buf.c | 2 ++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 120ef99d09e8..0d96efff451e 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -345,6 +345,10 @@ xfs_buf_allocate_memory(
 	unsigned short		page_count, i;
 	xfs_off_t		start, end;
 	int			error;
+	uint			kmflag_mask = 0;
+
+	if ((flags & XBF_ZERO) && !(flags & XBF_READ))
+		kmflag_mask |= KM_ZERO;
 
 	/*
 	 * for buffers that are contained within a single page, just allocate
@@ -354,7 +358,8 @@ xfs_buf_allocate_memory(
 	size = BBTOB(bp->b_length);
 	if (size < PAGE_SIZE) {
 		int align_mask = xfs_buftarg_dma_alignment(bp->b_target);
-		bp->b_addr = kmem_alloc_io(size, align_mask, KM_NOFS);
+		bp->b_addr = kmem_alloc_io(size, align_mask,
+					   KM_NOFS | kmflag_mask);
 		if (!bp->b_addr) {
 			/* low memory - use alloc_page loop instead */
 			goto use_alloc_page;
@@ -391,7 +396,7 @@ xfs_buf_allocate_memory(
 		struct page	*page;
 		uint		retries = 0;
 retry:
-		page = alloc_page(gfp_mask);
+		page = alloc_page(gfp_mask | kmflag_mask);
 		if (unlikely(page == NULL)) {
 			if (flags & XBF_READ_AHEAD) {
 				bp->b_page_count = i;
diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index f6ce17d8d848..dccdd653c9dc 100644
--- a/fs/xfs/xfs_buf.h
+++ b/fs/xfs/xfs_buf.h
@@ -33,6 +33,7 @@
 /* flags used only as arguments to access routines */
 #define XBF_TRYLOCK	 (1 << 16)/* lock requested, but do not wait */
 #define XBF_UNMAPPED	 (1 << 17)/* do not map the buffer */
+#define XBF_ZERO	 (1 << 18)/* zeroed buffer required */
 
 /* flags used only internally */
 #define _XBF_PAGES	 (1 << 20)/* backed by refcounted pages */
@@ -52,6 +53,7 @@ typedef unsigned int xfs_buf_flags_t;
 	{ XBF_WRITE_FAIL,	"WRITE_FAIL" }, \
 	{ XBF_TRYLOCK,		"TRYLOCK" },	/* should never be set */\
 	{ XBF_UNMAPPED,		"UNMAPPED" },	/* ditto */\
+	{ XBF_ZERO,		"KMEM_ZERO" }, \
 	{ _XBF_PAGES,		"PAGES" }, \
 	{ _XBF_KMEM,		"KMEM" }, \
 	{ _XBF_DELWRI_Q,	"DELWRI_Q" }
diff --git a/fs/xfs/xfs_trans_buf.c b/fs/xfs/xfs_trans_buf.c
index b5b3a78ef31c..087d413c1490 100644
--- a/fs/xfs/xfs_trans_buf.c
+++ b/fs/xfs/xfs_trans_buf.c
@@ -123,6 +123,8 @@ xfs_trans_get_buf_map(
 	xfs_buf_t		*bp;
 	struct xfs_buf_log_item	*bip;
 
+	flags |= XBF_ZERO;
+
 	if (!tp)
 		return xfs_buf_get_map(target, map, nmaps, flags);
 
-- 
2.21.0