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Subject[PATCH 4.17 48/66] staging: speakup: fix wraparound in uaccess length check
Date
4.17-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Samuel Thibault <samuel.thibault@ens-lyon.org>

commit b96fba8d5855c3617adbfb43ca4723a808cac954 upstream.

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: 425e586cf95b ("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>

---
drivers/staging/speakup/speakup_soft.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

--- a/drivers/staging/speakup/speakup_soft.c
+++ b/drivers/staging/speakup/speakup_soft.c
@@ -197,11 +197,15 @@ static ssize_t softsynthx_read(struct fi
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);
@@ -227,7 +231,7 @@ static ssize_t softsynthx_read(struct fi
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';

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