proc: fix UAF in proc_get_inode()

JIRA: https://issues.redhat.com/browse/RHEL-86809
CVE: CVE-2025-21999
Upstream status: Linus

Conflicts: There was fuzz 1 in hunk #1 against include/linux/proc_fs.h
	for which I can't see any visible difference.

commit 654b33a
Author: Ye Bin <yebin10@huawei.com>
Date:   Sat Mar 1 15:06:24 2025 +0300

    proc: fix UAF in proc_get_inode()

    Fix race between rmmod and /proc/XXX's inode instantiation.

    The bug is that pde->proc_ops don't belong to /proc, it belongs to a
    module, therefore dereferencing it after /proc entry has been registered
    is a bug unless use_pde/unuse_pde() pair has been used.

    use_pde/unuse_pde can be avoided (2 atomic ops!) because pde->proc_ops
    never changes so information necessary for inode instantiation can be
    saved _before_ proc_register() in PDE itself and used later, avoiding
    pde->proc_ops->...  dereference.

          rmmod                         lookup
    sys_delete_module
                             proc_lookup_de
                               pde_get(de);
                               proc_get_inode(dir->i_sb, de);
      mod->exit()
        proc_remove
          remove_proc_subtree
           proc_entry_rundown(de);
      free_module(mod);

                                   if (S_ISREG(inode->i_mode))
                                     if (de->proc_ops->proc_read_iter)
                               --> As module is already freed, will trigger UAF

    BUG: unable to handle page fault for address: fffffbfff80a702b
    PGD 817fc4067 P4D 817fc4067 PUD 817fc0067 PMD 102ef4067 PTE 0
    Oops: Oops: 0000 [#1] PREEMPT SMP KASAN PTI
    CPU: 26 UID: 0 PID: 2667 Comm: ls Tainted: G
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
    RIP: 0010:proc_get_inode+0x302/0x6e0
    RSP: 0018:ffff88811c837998 EFLAGS: 00010a06
    RAX: dffffc0000000000 RBX: ffffffffc0538140 RCX: 0000000000000007
    RDX: 1ffffffff80a702b RSI: 0000000000000001 RDI: ffffffffc0538158
    RBP: ffff8881299a6000 R08: 0000000067bbe1e5 R09: 1ffff11023906f20
    R10: ffffffffb560ca07 R11: ffffffffb2b43a58 R12: ffff888105bb78f0
    R13: ffff888100518048 R14: ffff8881299a6004 R15: 0000000000000001
    FS:  00007f95b9686840(0000) GS:ffff8883af100000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: fffffbfff80a702b CR3: 0000000117dd2000 CR4: 00000000000006f0
    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
    Call Trace:
     <TASK>
     proc_lookup_de+0x11f/0x2e0
     __lookup_slow+0x188/0x350
     walk_component+0x2ab/0x4f0
     path_lookupat+0x120/0x660
     filename_lookup+0x1ce/0x560
     vfs_statx+0xac/0x150
     __do_sys_newstat+0x96/0x110
     do_syscall_64+0x5f/0x170
     entry_SYSCALL_64_after_hwframe+0x76/0x7e

    [adobriyan@gmail.com: don't do 2 atomic ops on the common path]
    Link: https://lkml.kernel.org/r/3d25ded0-1739-447e-812b-e34da7990dcf@p183
    Fixes: 778f3dd ("Fix procfs compat_ioctl regression")
    Signed-off-by: Ye Bin <yebin10@huawei.com>
    Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: David S. Miller <davem@davemloft.net>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Ian Kent <ikent@redhat.com>

Author: Ian Kent

fs/proc/generic.c

@@ -558,10 +558,16 @@ struct proc_dir_entry *proc_create_reg(const char *name, umode_t mode,
 	return p;
 }
 
-static inline void pde_set_flags(struct proc_dir_entry *pde)
+static void pde_set_flags(struct proc_dir_entry *pde)
 {
 	if (pde->proc_ops->proc_flags & PROC_ENTRY_PERMANENT)
 		pde->flags |= PROC_ENTRY_PERMANENT;
+	if (pde->proc_ops->proc_read_iter)
+		pde->flags |= PROC_ENTRY_proc_read_iter;
+#ifdef CONFIG_COMPAT
+	if (pde->proc_ops->proc_compat_ioctl)
+		pde->flags |= PROC_ENTRY_proc_compat_ioctl;
+#endif
 }
 
 struct proc_dir_entry *proc_create_data(const char *name, umode_t mode,
@@ -625,6 +631,7 @@ struct proc_dir_entry *proc_create_seq_private(const char *name, umode_t mode,
 	p->proc_ops = &proc_seq_ops;
 	p->seq_ops = ops;
 	p->state_size = state_size;
+	pde_set_flags(p);
 	return proc_register(parent, p);
 }
 EXPORT_SYMBOL(proc_create_seq_private);
@@ -655,6 +662,7 @@ struct proc_dir_entry *proc_create_single_data(const char *name, umode_t mode,
 		return NULL;
 	p->proc_ops = &proc_single_ops;
 	p->single_show = show;
+	pde_set_flags(p);
 	return proc_register(parent, p);
 }
 EXPORT_SYMBOL(proc_create_single_data);

fs/proc/inode.c

@@ -672,13 +672,13 @@ struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
 
 	if (S_ISREG(inode->i_mode)) {
 		inode->i_op = de->proc_iops;
-		if (de->proc_ops->proc_read_iter)
+		if (pde_has_proc_read_iter(de))
 			inode->i_fop = &proc_iter_file_ops;
 		else
 			inode->i_fop = &proc_reg_file_ops;
 #ifdef CONFIG_COMPAT
-		if (de->proc_ops->proc_compat_ioctl) {
-			if (de->proc_ops->proc_read_iter)
+		if (pde_has_proc_compat_ioctl(de)) {
+			if (pde_has_proc_read_iter(de))
 				inode->i_fop = &proc_iter_file_ops_compat;
 			else
 				inode->i_fop = &proc_reg_file_ops_compat;

fs/proc/internal.h

@@ -79,6 +79,20 @@ static inline bool pde_is_permanent(const struct proc_dir_entry *pde)
 	return pde->flags & PROC_ENTRY_PERMANENT;
 }
 
+static inline bool pde_has_proc_read_iter(const struct proc_dir_entry *pde)
+{
+	return pde->flags & PROC_ENTRY_proc_read_iter;
+}
+
+static inline bool pde_has_proc_compat_ioctl(const struct proc_dir_entry *pde)
+{
+#ifdef CONFIG_COMPAT
+	return pde->flags & PROC_ENTRY_proc_compat_ioctl;
+#else
+	return false;
+#endif
+}
+
 extern struct kmem_cache *proc_dir_entry_cache;
 void pde_free(struct proc_dir_entry *pde);
 

include/linux/proc_fs.h

@@ -20,10 +20,13 @@ enum {
 	 * If in doubt, ignore this flag.
 	 */
 #ifdef MODULE
-	PROC_ENTRY_PERMANENT = 0U,
+	PROC_ENTRY_PERMANENT		= 0U,
 #else
-	PROC_ENTRY_PERMANENT = 1U << 0,
+	PROC_ENTRY_PERMANENT		= 1U << 0,
 #endif
+
+	PROC_ENTRY_proc_read_iter	= 1U << 1,
+	PROC_ENTRY_proc_compat_ioctl	= 1U << 2,
 };
 
 struct proc_ops {