Use 'strlen' of the string, add one for NUL terminator and simply do 'copy_to_user' instead of the explicit 'for' loop. This makes the KDGKBSENT case more compact.
The only thing we need to take care about is NULL 'func_table[i]'. Use an empty string in that case.
The original check for overflow could never trigger as the func_buf strings are always shorter or equal to 'struct kbsentry's.
Signed-off-by: Jiri Slaby jslaby@suse.cz Cc: stable@vger.kernel.org ---
[v2] Removed the need for "reorder user buffer handling in vt_do_kdgkb_ioctl" patch.
drivers/tty/vt/keyboard.c | 28 +++++++++------------------- 1 file changed, 9 insertions(+), 19 deletions(-)
diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c index 0db53b5b3acf..7bfa95c3252c 100644 --- a/drivers/tty/vt/keyboard.c +++ b/drivers/tty/vt/keyboard.c @@ -1995,9 +1995,7 @@ int vt_do_kdsk_ioctl(int cmd, struct kbentry __user *user_kbe, int perm, int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm) { struct kbsentry *kbs; - char *p; u_char *q; - u_char __user *up; int sz, fnw_sz; int delta; char *first_free, *fj, *fnw; @@ -2023,23 +2021,15 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm) i = array_index_nospec(kbs->kb_func, MAX_NR_FUNC);
switch (cmd) { - case KDGKBSENT: - sz = sizeof(kbs->kb_string) - 1; /* sz should have been - a struct member */ - up = user_kdgkb->kb_string; - p = func_table[i]; - if(p) - for ( ; *p && sz; p++, sz--) - if (put_user(*p, up++)) { - ret = -EFAULT; - goto reterr; - } - if (put_user('\0', up)) { - ret = -EFAULT; - goto reterr; - } - kfree(kbs); - return ((p && *p) ? -EOVERFLOW : 0); + case KDGKBSENT: { + /* size should have been a struct member */ + unsigned char *from = func_table[i] ? : ""; + + ret = copy_to_user(user_kdgkb->kb_string, from, + strlen(from) + 1) ? -EFAULT : 0; + + goto reterr; + } case KDSKBSENT: if (!perm) { ret = -EPERM;
Both read-side users of func_table/func_buf need locking. Without that, one can easily confuse the code by repeatedly setting altering strings like: while (1) for (a = 0; a < 2; a++) { struct kbsentry kbs = {}; strcpy((char *)kbs.kb_string, a ? ".\n" : "88888\n"); ioctl(fd, KDSKBSENT, &kbs); }
When that program runs, one can get unexpected output by holding F1 (note the unxpected period on the last line): . 88888 .8888
So protect all accesses to 'func_table' (and func_buf) by preexisting 'func_buf_lock'.
It is easy in 'k_fn' handler as 'puts_queue' is expected not to sleep. On the other hand, KDGKBSENT needs a local (atomic) copy of the string because copy_to_user can sleep. Use already allocated, but unused 'kbs->kb_string' for that purpose.
Note that the program above needs at least CAP_SYS_TTY_CONFIG.
This depends on the previous patch and on the func_buf_lock lock added in commit 46ca3f735f34 (tty/vt: fix write/write race in ioctl(KDSKBSENT) handler) in 5.2.
Likely fixes CVE-2020-25656.
Signed-off-by: Jiri Slaby jslaby@suse.cz Reported-by: Minh Yuan yuanmingbuaa@gmail.com Cc: stable@vger.kernel.org ---
[v2] - Removed the need for "reorder user buffer handling in vt_do_kdgkb_ioctl" patch. - given the above, use kbs->kb_string instead of another kmalloc.
drivers/tty/vt/keyboard.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c index 7bfa95c3252c..78acc270e39a 100644 --- a/drivers/tty/vt/keyboard.c +++ b/drivers/tty/vt/keyboard.c @@ -743,8 +743,13 @@ static void k_fn(struct vc_data *vc, unsigned char value, char up_flag) return;
if ((unsigned)value < ARRAY_SIZE(func_table)) { + unsigned long flags; + + spin_lock_irqsave(&func_buf_lock, flags); if (func_table[value]) puts_queue(vc, func_table[value]); + spin_unlock_irqrestore(&func_buf_lock, flags); + } else pr_err("k_fn called with value=%d\n", value); } @@ -1991,7 +1996,7 @@ int vt_do_kdsk_ioctl(int cmd, struct kbentry __user *user_kbe, int perm, #undef s #undef v
-/* FIXME: This one needs untangling and locking */ +/* FIXME: This one needs untangling */ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm) { struct kbsentry *kbs; @@ -2023,10 +2028,14 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm) switch (cmd) { case KDGKBSENT: { /* size should have been a struct member */ - unsigned char *from = func_table[i] ? : ""; + ssize_t len = sizeof(user_kdgkb->kb_string); + + spin_lock_irqsave(&func_buf_lock, flags); + len = strlcpy(kbs->kb_string, func_table[i] ? : "", len); + spin_unlock_irqrestore(&func_buf_lock, flags);
- ret = copy_to_user(user_kdgkb->kb_string, from, - strlen(from) + 1) ? -EFAULT : 0; + ret = copy_to_user(user_kdgkb->kb_string, kbs->kb_string, + len + 1) ? -EFAULT : 0;
goto reterr; }
linux-stable-mirror@lists.linaro.org
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Jiri Slaby