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Message #59190
[Bug 1314762] Re: n_tty_write crash when echoing in raw mode
** No longer affects: linux-armadaxp (Ubuntu)
** No longer affects: linux-armadaxp (Ubuntu)
** No longer affects: linux-ec2 (Ubuntu)
** No longer affects: linux-lts-quantal (Ubuntu)
** No longer affects: linux-lts-saucy (Ubuntu)
** No longer affects: linux-mvl-dove (Ubuntu)
** No longer affects: linux-mvl-dove (Ubuntu)
** No longer affects: linux-lts-saucy (Ubuntu)
** No longer affects: linux-lts-quantal (Ubuntu)
** No longer affects: linux-ec2 (Ubuntu)
** No longer affects: linux-armadaxp (Ubuntu)
** No longer affects: linux-mvl-dove (Ubuntu)
** No longer affects: linux-lts-saucy (Ubuntu)
** No longer affects: linux-lts-quantal (Ubuntu)
** No longer affects: linux-ec2 (Ubuntu)
** No longer affects: linux-armadaxp (Ubuntu)
** No longer affects: linux (Ubuntu)
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https://bugs.launchpad.net/bugs/1314762
Title:
n_tty_write crash when echoing in raw mode
Status in “linux-armadaxp” package in Ubuntu:
New
Status in “linux-ec2” package in Ubuntu:
New
Status in “linux-lts-quantal” package in Ubuntu:
New
Status in “linux-lts-saucy” package in Ubuntu:
New
Status in “linux-mvl-dove” package in Ubuntu:
New
Bug description:
The tty atomic_write_lock does not provide an exclusion guarantee for
the tty driver if the termios settings are LECHO & !OPOST. And since
it is unexpected and not allowed to call TTY buffer helpers like
tty_insert_flip_string concurrently, this may lead to crashes when
concurrect writers call pty_write. In that case the following two
writers:
* the ECHOing from a workqueue and
* pty_write from the process
race and can overflow the corresponding TTY buffer like follows.
If we look into tty_insert_flip_string_fixed_flag, there is:
int space = __tty_buffer_request_room(port, goal, flags);
struct tty_buffer *tb = port->buf.tail;
...
memcpy(char_buf_ptr(tb, tb->used), chars, space);
...
tb->used += space;
so the race of the two can result in something like this:
A B
__tty_buffer_request_room
__tty_buffer_request_room
memcpy(buf(tb->used), ...)
tb->used += space;
memcpy(buf(tb->used), ...) ->BOOM
B's memcpy is past the tty_buffer due to the previous A's tb->used
increment.
Since the N_TTY line discipline input processing can output
concurrently with a tty write, obtain the N_TTY ldisc output_lock to
serialize echo output with normal tty writes. This ensures the tty
buffer helper tty_insert_flip_string is not called concurrently and
everything is fine.
Note that this is nicely reproducible by an ordinary user using
forkpty and some setup around that (raw termios + ECHO). And it is
exploitable in kernels at least after commit
d945cb9cce20ac7143c2de8d88b187f62db99bdc (pty: Rework the pty layer to
use the normal buffering logic) in 2.6.31-rc3.
js: add more info to the commit log
js: switch to bool
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