syzkaller reported this UAF: BUG: KASAN: use-after-free in n_tty_receive_buf_common+0x2481/0x2940 drivers/tty/n_tty.c:1741 Read of size 1 at addr ffff8880089e40e9 by task syz-executor.1/13184 CPU: 0 PID: 13184 Comm: syz-executor.1 Not tainted 5.4.7 #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 Call Trace: ... kasan_report+0xe/0x20 mm/kasan/common.c:634 n_tty_receive_buf_common+0x2481/0x2940 drivers/tty/n_tty.c:1741 tty_ldisc_receive_buf+0xac/0x190 drivers/tty/tty_buffer.c:461 paste_selection+0x297/0x400 drivers/tty/vt/selection.c:372 tioclinux+0x20d/0x4e0 drivers/tty/vt/vt.c:3044 vt_ioctl+0x1bcf/0x28d0 drivers/tty/vt/vt_ioctl.c:364 tty_ioctl+0x525/0x15a0 drivers/tty/tty_io.c:2657 vfs_ioctl fs/ioctl.c:47 [inline] It is due to a race between parallel paste_selection (TIOCL_PASTESEL) and set_selection_user (TIOCL_SETSEL) invocations. One uses sel_buffer, while the other frees it and reallocates a new one for another selection. Add a mutex to close this race. The mutex takes care properly of sel_buffer and sel_buffer_lth only. The other selection global variables (like sel_start, sel_end, and sel_cons) are protected only in set_selection_user. The other functions need quite some more work to close the races of the variables there. This is going to happen later. This likely fixes (I am unsure as there is no reproducer provided) bug 206361 too. It was marked as CVE-2020-8648. Signed-off-by: Jiri Slaby <jslaby@xxxxxxx> Reported-by: syzbot+59997e8d5cbdc486e6f6@xxxxxxxxxxxxxxxxxxxxxxxxx References: https://bugzilla.kernel.org/show_bug.cgi?id=206361 --- drivers/tty/vt/selection.c | 23 +++++++++++++++++------ 1 file changed, 17 insertions(+), 6 deletions(-) diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c index 44d974d4159f..0c50d7410b31 100644 --- a/drivers/tty/vt/selection.c +++ b/drivers/tty/vt/selection.c @@ -16,6 +16,7 @@ #include <linux/tty.h> #include <linux/sched.h> #include <linux/mm.h> +#include <linux/mutex.h> #include <linux/slab.h> #include <linux/types.h> @@ -45,6 +46,7 @@ static volatile int sel_start = -1; /* cleared by clear_selection */ static int sel_end; static int sel_buffer_lth; static char *sel_buffer; +static DEFINE_MUTEX(sel_lock); /* clear_selection, highlight and highlight_pointer can be called from interrupt (via scrollback/front) */ @@ -186,7 +188,7 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty) char *bp, *obp; int i, ps, pe, multiplier; u32 c; - int mode; + int mode, ret = 0; poke_blanked_console(); @@ -212,6 +214,7 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty) if (ps > pe) /* make sel_start <= sel_end */ swap(ps, pe); + mutex_lock(&sel_lock); if (sel_cons != vc_cons[fg_console].d) { clear_selection(); sel_cons = vc_cons[fg_console].d; @@ -257,9 +260,10 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty) break; case TIOCL_SELPOINTER: highlight_pointer(pe); - return 0; + goto unlock; default: - return -EINVAL; + ret = -EINVAL; + goto unlock; } /* remove the pointer */ @@ -281,7 +285,7 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty) else if (new_sel_start == sel_start) { if (new_sel_end == sel_end) /* no action required */ - return 0; + goto unlock; else if (new_sel_end > sel_end) /* extend to right */ highlight(sel_end + 2, new_sel_end); else /* contract from right */ @@ -309,7 +313,8 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty) if (!bp) { printk(KERN_WARNING "selection: kmalloc() failed\n"); clear_selection(); - return -ENOMEM; + ret = -ENOMEM; + goto unlock; } kfree(sel_buffer); sel_buffer = bp; @@ -334,7 +339,9 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty) } } sel_buffer_lth = bp - sel_buffer; - return 0; +unlock: + mutex_unlock(&sel_lock); + return ret; } EXPORT_SYMBOL_GPL(set_selection_kernel); @@ -364,6 +371,7 @@ int paste_selection(struct tty_struct *tty) tty_buffer_lock_exclusive(&vc->port); add_wait_queue(&vc->paste_wait, &wait); + mutex_lock(&sel_lock); while (sel_buffer && sel_buffer_lth > pasted) { set_current_state(TASK_INTERRUPTIBLE); if (signal_pending(current)) { @@ -371,7 +379,9 @@ int paste_selection(struct tty_struct *tty) break; } if (tty_throttled(tty)) { + mutex_unlock(&sel_lock); schedule(); + mutex_lock(&sel_lock); continue; } __set_current_state(TASK_RUNNING); @@ -380,6 +390,7 @@ int paste_selection(struct tty_struct *tty) count); pasted += count; } + mutex_unlock(&sel_lock); remove_wait_queue(&vc->paste_wait, &wait); __set_current_state(TASK_RUNNING); -- 2.25.0
