[PATCH v2] misc: fastrpc: avoid duplicate DMA mappings in fastrpc_create_maps()

[PATCH v2] misc: fastrpc: avoid duplicate DMA mappings in fastrpc_create_maps()

[Jianping Li]

DMA handles passed as invoke arguments (scalars beyond nbufs) may refer
to the same dma_buf fd as an input/output buffer argument. Taking an
extra reference for such DMA handle maps leads to duplicate mappings and
an unbalanced reference count, since DMA handle maps are released
separately when the DSP returns the fd through the fdlist.

Fix this by not taking an extra reference for DMA handle arguments
(take_ref = false) and tagging them with FASTRPC_MAP_DMA_HANDLE. As
these maps are borrowed references, fastrpc_get_args() re-validates the
map via fastrpc_map_lookup() before dereferencing it, so it is not used
after being freed. fastrpc_put_args() only releases maps flagged as
FASTRPC_MAP_DMA_HANDLE and clears the flag to guarantee the map is freed
exactly once.

Also reject FASTRPC_MAP_DMA_HANDLE in fastrpc_req_mem_map(), since such
handles are already mapped implicitly during the remote invoke call and
must not be mapped again through the explicit MEM_MAP path.

Fixes: 10df039834f84 ("misc: fastrpc: Skip reference for DMA handles")
Cc: stable@kernel.org
Signed-off-by: Jianping Li <jianping.li@oss.qualcomm.com>
---
Patch [v1]: https://lore.kernel.org/all/20260625080832.17477-1-jianping.li@oss.qualcomm.com/

Changes in v2:
- Rework the commit message to describe the DMA handle reference and
  lifetime problem more precisely.
- Introduce a new FASTRPC_MAP_DMA_HANDLE uapi flag and a 'flags' field
  in struct fastrpc_map to explicitly tag DMA handle maps, instead of
  relying only on the nbufs boundary / take_ref.
- Plumb an mflags argument through fastrpc_map_create() and
  fastrpc_map_attach() so DMA handle maps are tagged at creation time.
- Re-validate the borrowed map in fastrpc_get_args() via
  fastrpc_map_lookup() before dereferencing it, to avoid a
  use-after-free when the map was created with take_ref = false.
- In fastrpc_put_args(), only release maps tagged FASTRPC_MAP_DMA_HANDLE
  and clear the flag afterwards, so such maps are freed exactly once.
- Reject FASTRPC_MAP_DMA_HANDLE in fastrpc_req_mem_map(), since these
  handles are already mapped implicitly during the remote invoke and
  must not be mapped again through the explicit MEM_MAP path.
---
 drivers/misc/fastrpc.c      | 56 ++++++++++++++++++++++++++-----------
 include/uapi/misc/fastrpc.h |  2 ++
 2 files changed, 42 insertions(+), 16 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index d86e79134c68..fa8c58a97e35 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -223,6 +223,7 @@ struct fastrpc_map {
 	u64 len;
 	u64 raddr;
 	u32 attr;
+	u32 flags;
 	struct kref refcount;
 };
 
@@ -833,7 +834,7 @@ static dma_addr_t fastrpc_compute_dma_addr(struct fastrpc_user *fl, dma_addr_t s
 }
 
 static int fastrpc_map_attach(struct fastrpc_user *fl, int fd,
-			      u64 len, u32 attr, struct fastrpc_map **ppmap)
+			      u64 len, u32 attr, struct fastrpc_map **ppmap, int mflags)
 {
 	struct fastrpc_session_ctx *sess = fl->sctx;
 	struct fastrpc_map *map = NULL;
@@ -850,6 +851,7 @@ static int fastrpc_map_attach(struct fastrpc_user *fl, int fd,
 
 	map->fl = fl;
 	map->fd = fd;
+	map->flags = mflags;
 	map->buf = dma_buf_get(fd);
 	if (IS_ERR(map->buf)) {
 		err = PTR_ERR(map->buf);
@@ -924,13 +926,13 @@ static int fastrpc_map_attach(struct fastrpc_user *fl, int fd,
 	return err;
 }
 
-static int fastrpc_map_create(struct fastrpc_user *fl, int fd,
-			      u64 len, u32 attr, struct fastrpc_map **ppmap)
+static int fastrpc_map_create(struct fastrpc_user *fl, int fd, u64 len, u32 attr,
+			      struct fastrpc_map **ppmap, bool take_ref, int mflags)
 {
-	if (!fastrpc_map_lookup(fl, fd, ppmap, true))
+	if (!fastrpc_map_lookup(fl, fd, ppmap, take_ref))
 		return 0;
 
-	return fastrpc_map_attach(fl, fd, len, attr, ppmap);
+	return fastrpc_map_attach(fl, fd, len, attr, ppmap, mflags);
 }
 
 /*
@@ -1000,23 +1002,25 @@ static int fastrpc_create_maps(struct fastrpc_invoke_ctx *ctx)
 	int i, err;
 
 	for (i = 0; i < ctx->nscalars; ++i) {
+		bool take_ref = i < ctx->nbufs;
+		int mflags = 0;
 
 		if (ctx->args[i].fd == 0 || ctx->args[i].fd == -1 ||
 		    ctx->args[i].length == 0)
 			continue;
 
-		if (i < ctx->nbufs)
-			err = fastrpc_map_create(ctx->fl, ctx->args[i].fd,
-				 ctx->args[i].length, ctx->args[i].attr, &ctx->maps[i]);
-		else
-			err = fastrpc_map_attach(ctx->fl, ctx->args[i].fd,
-				 ctx->args[i].length, ctx->args[i].attr, &ctx->maps[i]);
+		/* Set the DMA handle mapping flag for DMA handles */
+		if (i >= ctx->nbufs)
+			mflags = FASTRPC_MAP_DMA_HANDLE;
+
+		err = fastrpc_map_create(ctx->fl, ctx->args[i].fd, ctx->args[i].length,
+					 ctx->args[i].attr, &ctx->maps[i], take_ref, mflags);
 		if (err) {
 			dev_err(dev, "Error Creating map %d\n", err);
 			return -EINVAL;
 		}
-
 	}
+
 	return 0;
 }
 
@@ -1144,6 +1148,16 @@ static int fastrpc_get_args(u32 kernel, struct fastrpc_invoke_ctx *ctx)
 		list[i].num = ctx->args[i].length ? 1 : 0;
 		list[i].pgidx = i;
 		if (ctx->maps[i]) {
+			/* It is possible that map is created with
+			 * mflags FASTRPC_MAP_DMA_HANDLE and take_ref
+			 * is false. Check if map still exists or is
+			 * being freed as take_ref is false
+			 */
+			if (fastrpc_map_lookup(ctx->fl, ctx->args[i].fd,
+					       &ctx->maps[i], false)) {
+				ctx->maps[i] = NULL;
+				return -EINVAL;
+			}
 			pages[i].addr = ctx->maps[i]->dma_addr;
 			pages[i].size = ctx->maps[i]->size;
 		}
@@ -1200,8 +1214,13 @@ static int fastrpc_put_args(struct fastrpc_invoke_ctx *ctx,
 	for (i = 0; i < FASTRPC_MAX_FDLIST; i++) {
 		if (!fdlist[i])
 			break;
-		if (!fastrpc_map_lookup(fl, (int)fdlist[i], &mmap, false))
+		/* Validate the map flags for DMA handles and skip freeing map if invalid */
+		if (!fastrpc_map_lookup(fl, (int)fdlist[i], &mmap, false) &&
+		    mmap->flags == FASTRPC_MAP_DMA_HANDLE) {
+			/* Allow DMA handle maps to free only once */
+			mmap->flags = 0;
 			fastrpc_map_put(mmap);
+		}
 	}
 
 	return ret;
@@ -1511,7 +1530,7 @@ static int fastrpc_init_create_process(struct fastrpc_user *fl,
 	fl->pd = USER_PD;
 
 	if (init.filelen && init.filefd) {
-		err = fastrpc_map_create(fl, init.filefd, init.filelen, 0, &map);
+		err = fastrpc_map_create(fl, init.filefd, init.filelen, 0, &map, true, 0);
 		if (err)
 			goto err;
 	}
@@ -2107,9 +2126,14 @@ static int fastrpc_req_mem_map(struct fastrpc_user *fl, char __user *argp)
 
 	if (copy_from_user(&req, argp, sizeof(req)))
 		return -EFAULT;
-
+	/*
+	 * Prevent mapping backward compatible DMA handles here, as they are
+	 * already mapped in the remote call.
+	 */
+	if (req.flags == FASTRPC_MAP_DMA_HANDLE)
+		return -EINVAL;
 	/* create SMMU mapping */
-	err = fastrpc_map_create(fl, req.fd, req.length, 0, &map);
+	err = fastrpc_map_create(fl, req.fd, req.length, 0, &map, true, 0);
 	if (err) {
 		dev_err(dev, "failed to map buffer, fd = %d\n", req.fd);
 		return err;
diff --git a/include/uapi/misc/fastrpc.h b/include/uapi/misc/fastrpc.h
index c6e2925f47e6..142ddaeed85f 100644
--- a/include/uapi/misc/fastrpc.h
+++ b/include/uapi/misc/fastrpc.h
@@ -44,6 +44,8 @@ enum fastrpc_map_flags {
 	FASTRPC_MAP_FD = 2,
 	FASTRPC_MAP_FD_DELAYED,
 	FASTRPC_MAP_FD_NOMAP = 16,
+	/* Map the DMA handle in the invoke call for backward compatibility */
+	FASTRPC_MAP_DMA_HANDLE = 0x20000,
 	FASTRPC_MAP_MAX,
 };
 
-- 
2.43.0