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staging: speakup: fix wraparound in uaccess length check

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If softsynthx_read() is called with `count < 3`, `count - 3` wraps, causing
the loop to copy as much data as available to the provided buffer. If
softsynthx_read() is invoked through sys_splice(), this causes an
unbounded kernel write; but even when userspace just reads from it
normally, a small size could cause userspace crashes.

Fixes: 425e586cf9 ("speakup: add unicode variant of /dev/softsynth")
Cc: stable@vger.kernel.org
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Samuel Thibault 2018-07-13 00:29:36 +02:00 committed by Greg Kroah-Hartman
parent eb37430d40
commit b96fba8d58
1 changed files with 5 additions and 1 deletions
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6
drivers/staging/speakup/speakup_soft.c
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@ -198,11 +198,15 @@ static ssize_t softsynthx_read(struct file *fp, char __user *buf, size_t count,
int chars_sent = 0;
char __user *cp;
char *init;
size_t bytes_per_ch = unicode ? 3 : 1;
u16 ch;
int empty;
unsigned long flags;
DEFINE_WAIT(wait);
if (count < bytes_per_ch)
return -EINVAL;
spin_lock_irqsave(&speakup_info.spinlock, flags);
while (1) {
prepare_to_wait(&speakup_event, &wait, TASK_INTERRUPTIBLE);
@ -228,7 +232,7 @@ static ssize_t softsynthx_read(struct file *fp, char __user *buf, size_t count,
init = get_initstring();
/* Keep 3 bytes available for a 16bit UTF-8-encoded character */
while (chars_sent <= count - 3) {
while (chars_sent <= count - bytes_per_ch) {
if (speakup_info.flushing) {
speakup_info.flushing = 0;
ch = '\x18';