[PATCH] io_uring: use current task creds instead of allocating a new one

[PATCH] io_uring: use current task creds instead of allocating a new one

[Jens Axboe]

syzbot reports:

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 9217 Comm: io_uring-sq Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:creds_are_invalid kernel/cred.c:792 [inline]
RIP: 0010:__validate_creds include/linux/cred.h:187 [inline]
RIP: 0010:override_creds+0x9f/0x170 kernel/cred.c:550
Code: ac 25 00 81 fb 64 65 73 43 0f 85 a3 37 00 00 e8 17 ab 25 00 49 8d 7c
24 10 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84
c0 74 08 3c 03 0f 8e 96 00 00 00 41 8b 5c 24 10 bf
RSP: 0018:ffff88809c45fda0 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000043736564 RCX: ffffffff814f3318
RDX: 0000000000000002 RSI: ffffffff814f3329 RDI: 0000000000000010
RBP: ffff88809c45fdb8 R08: ffff8880a3aac240 R09: ffffed1014755849
R10: ffffed1014755848 R11: ffff8880a3aac247 R12: 0000000000000000
R13: ffff888098ab1600 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffd51c40664 CR3: 0000000092641000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
   io_sq_thread+0x1c7/0xa20 fs/io_uring.c:3274
   kthread+0x361/0x430 kernel/kthread.c:255
   ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Modules linked in:
---[ end trace f2e1a4307fbe2245 ]---
RIP: 0010:creds_are_invalid kernel/cred.c:792 [inline]
RIP: 0010:__validate_creds include/linux/cred.h:187 [inline]
RIP: 0010:override_creds+0x9f/0x170 kernel/cred.c:550
Code: ac 25 00 81 fb 64 65 73 43 0f 85 a3 37 00 00 e8 17 ab 25 00 49 8d 7c
24 10 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84
c0 74 08 3c 03 0f 8e 96 00 00 00 41 8b 5c 24 10 bf
RSP: 0018:ffff88809c45fda0 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000043736564 RCX: ffffffff814f3318
RDX: 0000000000000002 RSI: ffffffff814f3329 RDI: 0000000000000010
RBP: ffff88809c45fdb8 R08: ffff8880a3aac240 R09: ffffed1014755849
R10: ffffed1014755848 R11: ffff8880a3aac247 R12: 0000000000000000
R13: ffff888098ab1600 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffd51c40664 CR3: 0000000092641000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

which is caused by slab fault injection triggering a failure in
prepare_creds(). We don't actually need to create a copy of the creds
as we're not modifying it, we just need a reference on the current task
creds. This avoids the failure case as well, and propagates the const
throughout the stack.

Fixes: 181e448d8709 ("io_uring: async workers should inherit the user creds")
Reported-by: syzbot+5320383e16029ba057ff@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 91b85df0861e..74b40506c5d9 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -111,7 +111,7 @@ struct io_wq {
  
  	struct task_struct *manager;
  	struct user_struct *user;
-	struct cred *creds;
+	const struct cred *creds;
  	struct mm_struct *mm;
  	refcount_t refs;
  	struct completion done;
diff --git a/fs/io-wq.h b/fs/io-wq.h
index 600e0158cba7..dd0af0d7376c 100644
--- a/fs/io-wq.h
+++ b/fs/io-wq.h
@@ -87,7 +87,7 @@ typedef void (put_work_fn)(struct io_wq_work *);
  struct io_wq_data {
  	struct mm_struct *mm;
  	struct user_struct *user;
-	struct cred *creds;
+	const struct cred *creds;
  
  	get_work_fn *get_work;
  	put_work_fn *put_work;
diff --git a/fs/io_uring.c b/fs/io_uring.c
index ec53aa7cdc94..5cab7a047317 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -238,7 +238,7 @@ struct io_ring_ctx {
  
  	struct user_struct	*user;
  
-	struct cred		*creds;
+	const struct cred	*creds;
  
  	/* 0 is for ctx quiesce/reinit/free, 1 is for sqo_thread started */
  	struct completion	*completions;
@@ -4759,7 +4759,7 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
  	ctx->compat = in_compat_syscall();
  	ctx->account_mem = account_mem;
  	ctx->user = user;
-	ctx->creds = prepare_creds();
+	ctx->creds = get_current_cred();
  
  	ret = io_allocate_scq_urings(ctx, p);
  	if (ret)

-- 
Jens Axboe