Re: How to handle concurrent access to /dev/ttyprintk ?
From: Tetsuo Handa
Date: Tue Apr 13 2021 - 20:45:14 EST
On 2021/04/12 21:04, Greg Kroah-Hartman wrote:
>> Since syzkaller is a fuzzer, syzkaller happily opens /dev/ttyprintk from
>> multiple threads. Should we update syzkaller to use CONFIG_TTY_PRINTK=n ?
>
> Why? Can you not hit the same tty code paths from any other tty driver
> being open multiple times? Why is ttyprintk somehow "special" here?
I found a simplified reproducer. If we call ioctl(TIOCVHANGUP) on /dev/ttyprintk ,
"struct ttyprintk_port tpk_port".port.count cannot be decremented by
tty_port_close() from tpk_close() due to tty_hung_up_p() == true when
close() is called. As a result, tty->count and port count gets out of sync.
Then, when /dev/ttyprintk is opened again and then closed without calling
ioctl(TIOCVHANGUP), this message is printed due to tty_hung_up_p() == false.
---------- Debug print patch start ----------
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 391bada4cedb..42d54368adac 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -113,7 +113,7 @@
#ifdef TTY_DEBUG_HANGUP
# define tty_debug_hangup(tty, f, args...) tty_debug(tty, f, ##args)
#else
-# define tty_debug_hangup(tty, f, args...) do { } while (0)
+# define tty_debug_hangup(tty, f, args...) tty_info(tty, f, ##args)
#endif
#define TTY_PARANOIA_CHECK 1
@@ -628,6 +628,7 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
closecount++;
__tty_fasync(-1, filp, 0); /* can't block */
filp->f_op = &hung_up_tty_fops;
+ tty_warn(tty, "Set hung_up_tty_fops on %px\n", filp);
}
spin_unlock(&tty->files_lock);
diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 346d20f4a486..032fc58203ea 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -561,8 +561,10 @@ int tty_port_close_start(struct tty_port *port,
{
unsigned long flags;
- if (tty_hung_up_p(filp))
+ if (tty_hung_up_p(filp)) {
+ tty_warn(tty, "Skipped by hung_up_tty_fops on %px\n", filp);
return 0;
+ }
spin_lock_irqsave(&port->lock, flags);
if (tty->count == 1 && port->count != 1) {
---------- Debug print patch end ----------
---------- Reproducer start ----------
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int i;
int fd[10];
for (i = 0; i < 10; i++)
fd[i] = open("/dev/ttyprintk", O_WRONLY);
ioctl(fd[0], TIOCVHANGUP);
for (i = 0; i < 10; i++)
close(fd[i]);
close(open("/dev/ttyprintk", O_WRONLY));
return 0;
}
---------- Reproducer end ----------
---------- Console output start ----------
[ 31.885820] ttyprintk ttyprintk: opening (count=1)
[ 31.885859] ttyprintk ttyprintk: opening (count=2)
[ 31.885876] ttyprintk ttyprintk: opening (count=3)
[ 31.885892] ttyprintk ttyprintk: opening (count=4)
[ 31.885907] ttyprintk ttyprintk: opening (count=5)
[ 31.885923] ttyprintk ttyprintk: opening (count=6)
[ 31.885939] ttyprintk ttyprintk: opening (count=7)
[ 31.885956] ttyprintk ttyprintk: opening (count=8)
[ 31.885971] ttyprintk ttyprintk: opening (count=9)
[ 31.885988] ttyprintk ttyprintk: opening (count=10)
[ 31.885999] ttyprintk ttyprintk: vhangup
[ 31.886005] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed1000
[ 31.886009] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed2000
[ 31.886012] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed0c00
[ 31.886016] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed0e00
[ 31.886019] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed2c00
[ 31.886023] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed3a00
[ 31.886026] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed3400
[ 31.886029] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed3c00
[ 31.886033] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed0200
[ 31.886036] ttyprintk ttyprintk: Set hung_up_tty_fops on ffff97c38eed3e00
[ 31.886055] ttyprintk ttyprintk: releasing (count=10)
[ 31.886106] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed3e00
[ 31.886117] ttyprintk ttyprintk: releasing (count=9)
[ 31.886121] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed0200
[ 31.886128] ttyprintk ttyprintk: releasing (count=8)
[ 31.886131] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed3c00
[ 31.886138] ttyprintk ttyprintk: releasing (count=7)
[ 31.886141] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed3400
[ 31.886148] ttyprintk ttyprintk: releasing (count=6)
[ 31.886151] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed3a00
[ 31.886158] ttyprintk ttyprintk: releasing (count=5)
[ 31.886161] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed2c00
[ 31.886168] ttyprintk ttyprintk: releasing (count=4)
[ 31.886171] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed0e00
[ 31.886178] ttyprintk ttyprintk: releasing (count=3)
[ 31.886181] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed0c00
[ 31.886188] ttyprintk ttyprintk: releasing (count=2)
[ 31.886191] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed2000
[ 31.886198] ttyprintk ttyprintk: releasing (count=1)
[ 31.886201] ttyprintk ttyprintk: Skipped by hung_up_tty_fops on ffff97c38eed1000
[ 31.886205] ttyprintk ttyprintk: final close
[ 31.886211] ttyprintk ttyprintk: freeing structure
[ 31.886277] ttyprintk ttyprintk: opening (count=1)
[ 31.886531] ttyprintk ttyprintk: releasing (count=1)
[ 31.886535] ttyprintk ttyprintk: tty_port_close_start: tty->count = 1 port count = 11
[ 31.886543] ttyprintk ttyprintk: final close
[ 31.886552] ttyprintk ttyprintk: freeing structure
---------- Console output end ----------
If I replace /dev/ttyprintk with /dev/ttyS0 in the reproducer shown above,
this message is not printed.
---------- Console output start ----------
[ 10.794802] ttyS ttyS0: opening (count=1)
[ 10.795851] ttyS ttyS0: opening (count=2)
[ 10.795872] ttyS ttyS0: opening (count=3)
[ 10.795890] ttyS ttyS0: opening (count=4)
[ 10.795908] ttyS ttyS0: opening (count=5)
[ 10.795925] ttyS ttyS0: opening (count=6)
[ 10.795943] ttyS ttyS0: opening (count=7)
[ 10.795960] ttyS ttyS0: opening (count=8)
[ 10.795978] ttyS ttyS0: opening (count=9)
[ 10.795995] ttyS ttyS0: opening (count=10)
[ 10.796007] ttyS ttyS0: vhangup
[ 10.796014] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc5600
[ 10.796020] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc6400
[ 10.796025] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc4a00
[ 10.796030] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc6800
[ 10.796035] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc6200
[ 10.796039] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc5a00
[ 10.796044] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc6600
[ 10.796048] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc7600
[ 10.796053] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc5e00
[ 10.796058] ttyS ttyS0: Set hung_up_tty_fops on ffff9cb044dc6000
[ 10.796401] ttyS ttyS0: releasing (count=10)
[ 10.796408] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc6000
[ 10.796434] ttyS ttyS0: releasing (count=9)
[ 10.796439] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc5e00
[ 10.796447] ttyS ttyS0: releasing (count=8)
[ 10.796451] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc7600
[ 10.796459] ttyS ttyS0: releasing (count=7)
[ 10.796463] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc6600
[ 10.796472] ttyS ttyS0: releasing (count=6)
[ 10.796476] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc5a00
[ 10.796484] ttyS ttyS0: releasing (count=5)
[ 10.796488] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc6200
[ 10.796496] ttyS ttyS0: releasing (count=4)
[ 10.796500] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc6800
[ 10.796508] ttyS ttyS0: releasing (count=3)
[ 10.796512] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc4a00
[ 10.796521] ttyS ttyS0: releasing (count=2)
[ 10.796525] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc6400
[ 10.796533] ttyS ttyS0: releasing (count=1)
[ 10.796537] ttyS ttyS0: Skipped by hung_up_tty_fops on ffff9cb044dc5600
[ 10.796542] ttyS ttyS0: final close
[ 10.796548] ttyS ttyS0: freeing structure
[ 10.796634] ttyS ttyS0: opening (count=1)
[ 10.797192] ttyS ttyS0: releasing (count=1)
[ 10.797413] ttyS ttyS0: final close
[ 10.797427] ttyS ttyS0: freeing structure
---------- Console output end ----------