[PATCH] kernfs: protect of->kn access in fop_read_iter/fop_mmap

[Ravineet Singh <ravineet.a.singh@est.tech>]

kernfs_fop_read_iter() and kernfs_fop_mmap() dereference of->kn->flags
without holding an active reference on the kernfs node. If the node is
removed concurrently, this leads to a use-after-free:

[  448.037888] Unable to handle kernel paging request at virtual address ffffff821d8cedf0
[  448.093213] Mem abort info:
[  448.104535]   ESR = 0x0000000096000005
[  448.113391]   EC = 0x25: DABT (current EL), IL = 32 bits
[  448.126411]   SET = 0, FnV = 0
[  448.130758]   EA = 0, S1PTW = 0
[  448.134268]   FSC = 0x05: level 1 translation fault
[  448.140335] Data abort info:
[  448.143275]   ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
[  448.150223]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[  448.155668]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[  448.161233] swapper pgtable: 4k pages, 39-bit VAs, pgdp=00000000afab8000
[  448.168835] [ffffff821d8cedf0] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
[  448.178817] Internal error: Oops: 0000000096000005 [#1] PREEMPT SMP
[  448.284717] pc : kernfs_fop_read_iter+0x1c/0x1ac
[  448.289416] lr : vfs_read+0x1c0/0x2a0
[  448.368374] Call trace:
[  448.370855]  kernfs_fop_read_iter+0x1c/0x1ac
[  448.375156]  vfs_read+0x1c0/0x2a0
[  448.378508]  ksys_read+0x6c/0x100
[  448.381901]  __arm64_sys_read+0x18/0x20
[  448.385768]  invoke_syscall.constprop.0+0x4c/0xe0
[  448.390502]  do_el0_svc+0x3c/0xb8
[  448.393898]  el0_svc+0x18/0x4c
[  448.396990]  el0t_64_sync_handler+0x118/0x124
[  448.401377]  el0t_64_sync+0x14c/0x150

Use kernfs_get_active_of() to obtain an active reference that also
checks the released flag, consistent with other of->kn accesses in
fs/kernfs/file.c. These paths were not covered when
kernfs_get_active_of() was introduced in commit 3c9ba2777d6c8
("kernfs: Fix UAF in polling when open file is released").

Fixes: 4eaad21a6ac9 ("kernfs: implement ->read_iter")
Cc: stable <stable@kernel.org>
Signed-off-by: Ravineet Singh <ravineet.a.singh@est.tech>
Signed-off-by: David Nyström <david.nystrom@est.tech>
---
 fs/kernfs/file.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 1163aa7697384..6a52d24a63156 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -293,7 +293,16 @@ static ssize_t kernfs_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 
 static ssize_t kernfs_fop_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 {
-	if (kernfs_of(iocb->ki_filp)->kn->flags & KERNFS_HAS_SEQ_SHOW)
+	struct kernfs_open_file *of = kernfs_of(iocb->ki_filp);
+	bool has_seq;
+
+	if (!kernfs_get_active_of(of))
+		return -ENODEV;
+
+	has_seq = of->kn->flags & KERNFS_HAS_SEQ_SHOW;
+	kernfs_put_active_of(of);
+
+	if (has_seq)
 		return seq_read_iter(iocb, iter);
 	return kernfs_file_read_iter(iocb, iter);
 }
@@ -458,6 +467,7 @@ static int kernfs_fop_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct kernfs_open_file *of = kernfs_of(file);
 	const struct kernfs_ops *ops;
+	bool has_mmap;
 	int rc;
 
 	/*
@@ -467,7 +477,13 @@ static int kernfs_fop_mmap(struct file *file, struct vm_area_struct *vma)
 	 * without grabbing @of->mutex by testing HAS_MMAP flag.  See the
 	 * comment in kernfs_fop_open() for more details.
 	 */
-	if (!(of->kn->flags & KERNFS_HAS_MMAP))
+	if (!kernfs_get_active_of(of))
+		return -ENODEV;
+
+	has_mmap = of->kn->flags & KERNFS_HAS_MMAP;
+	kernfs_put_active_of(of);
+
+	if (!has_mmap)
 		return -ENODEV;
 
 	mutex_lock(&of->mutex);
-- 
2.43.0