Patch "vc_screen: reload load of struct vc_data pointer in vcs_write() to avoid UAF" has been added to the 5.4-stable tree

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This is a note to let you know that I've just added the patch titled

    vc_screen: reload load of struct vc_data pointer in vcs_write() to avoid UAF

to the 5.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     vc_screen-reload-load-of-struct-vc_data-pointer-in-v.patch
and it can be found in the queue-5.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit e50829937cbc170854c92f65775fb4d3835c946a
Author: George Kennedy <george.kennedy@xxxxxxxxxx>
Date:   Fri May 12 06:08:48 2023 -0500

    vc_screen: reload load of struct vc_data pointer in vcs_write() to avoid UAF
    
    [ Upstream commit 8fb9ea65c9d1338b0d2bb0a9122dc942cdd32357 ]
    
    After a call to console_unlock() in vcs_write() the vc_data struct can be
    freed by vc_port_destruct(). Because of that, the struct vc_data pointer
    must be reloaded in the while loop in vcs_write() after console_lock() to
    avoid a UAF when vcs_size() is called.
    
    Syzkaller reported a UAF in vcs_size().
    
    BUG: KASAN: slab-use-after-free in vcs_size (drivers/tty/vt/vc_screen.c:215)
    Read of size 4 at addr ffff8880beab89a8 by task repro_vcs_size/4119
    
    Call Trace:
     <TASK>
    __asan_report_load4_noabort (mm/kasan/report_generic.c:380)
    vcs_size (drivers/tty/vt/vc_screen.c:215)
    vcs_write (drivers/tty/vt/vc_screen.c:664)
    vfs_write (fs/read_write.c:582 fs/read_write.c:564)
    ...
     <TASK>
    
    Allocated by task 1213:
    kmalloc_trace (mm/slab_common.c:1064)
    vc_allocate (./include/linux/slab.h:559 ./include/linux/slab.h:680
        drivers/tty/vt/vt.c:1078 drivers/tty/vt/vt.c:1058)
    con_install (drivers/tty/vt/vt.c:3334)
    tty_init_dev (drivers/tty/tty_io.c:1303 drivers/tty/tty_io.c:1415
        drivers/tty/tty_io.c:1392)
    tty_open (drivers/tty/tty_io.c:2082 drivers/tty/tty_io.c:2128)
    chrdev_open (fs/char_dev.c:415)
    do_dentry_open (fs/open.c:921)
    vfs_open (fs/open.c:1052)
    ...
    
    Freed by task 4116:
    kfree (mm/slab_common.c:1016)
    vc_port_destruct (drivers/tty/vt/vt.c:1044)
    tty_port_destructor (drivers/tty/tty_port.c:296)
    tty_port_put (drivers/tty/tty_port.c:312)
    vt_disallocate_all (drivers/tty/vt/vt_ioctl.c:662 (discriminator 2))
    vt_ioctl (drivers/tty/vt/vt_ioctl.c:903)
    tty_ioctl (drivers/tty/tty_io.c:2778)
    ...
    
    The buggy address belongs to the object at ffff8880beab8800
     which belongs to the cache kmalloc-1k of size 1024
    The buggy address is located 424 bytes inside of
     freed 1024-byte region [ffff8880beab8800, ffff8880beab8c00)
    
    The buggy address belongs to the physical page:
    page:00000000afc77580 refcount:1 mapcount:0 mapping:0000000000000000
        index:0x0 pfn:0xbeab8
    head:00000000afc77580 order:3 entire_mapcount:0 nr_pages_mapped:0
        pincount:0
    flags: 0xfffffc0010200(slab|head|node=0|zone=1|lastcpupid=0x1fffff)
    page_type: 0xffffffff()
    raw: 000fffffc0010200 ffff888100042dc0 ffffea000426de00 dead000000000002
    raw: 0000000000000000 0000000000100010 00000001ffffffff 0000000000000000
    page dumped because: kasan: bad access detected
    
    Memory state around the buggy address:
     ffff8880beab8880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
     ffff8880beab8900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
    >ffff8880beab8980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                      ^
     ffff8880beab8a00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
     ffff8880beab8a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
    ==================================================================
    Disabling lock debugging due to kernel taint
    
    Fixes: ac751efa6a0d ("console: rename acquire/release_console_sem() to console_lock/unlock()")
    Cc: stable <stable@xxxxxxxxxx>
    Reported-by: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
    Signed-off-by: George Kennedy <george.kennedy@xxxxxxxxxx>
    Reviewed-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/1683889728-10411-1-git-send-email-george.kennedy@xxxxxxxxxx
    Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index 48d74269f1d59..a6813e3393ece 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -523,10 +523,17 @@ vcs_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
 			}
 		}
 
-		/* The vcs_size might have changed while we slept to grab
-		 * the user buffer, so recheck.
+		/* The vc might have been freed or vcs_size might have changed
+		 * while we slept to grab the user buffer, so recheck.
 		 * Return data written up to now on failure.
 		 */
+		vc = vcs_vc(inode, &viewed);
+		if (!vc) {
+			if (written)
+				break;
+			ret = -ENXIO;
+			goto unlock_out;
+		}
 		size = vcs_size(vc, attr, false);
 		if (size < 0) {
 			if (written)



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