[PATCH] staging: speakup: fix wraparound in uaccess length check

[Jann Horn <jannh@google.com>]

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: Jann Horn <jannh@google.com>
---

Reproducer (kernel overflows userspace stack, resulting in segfault):

root@debian:/home/user# cat test.c
#include <unistd.h>
int main(void) {
  char buf[1];
  read(0, buf, 1);
}
root@debian:/home/user# gcc -o test test.c
root@debian:/home/user# ./test < /dev/softsynth
[do some stuff on the console so that it prints text]
Segmentation fault
root@debian:/home/user# strace ./test < /dev/softsynth
execve("./test", ["./test"], [/* 21 vars */]) = 0
brk(NULL)                               = 0x55d5977da000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f2ca2cac000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=103509, ...}) = 0
mmap(NULL, 103509, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f2ca2c92000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\3\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1689360, ...}) = 0
mmap(NULL, 3795360, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f2ca26ed000
mprotect(0x7f2ca2882000, 2097152, PROT_NONE) = 0
mmap(0x7f2ca2a82000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x195000) = 0x7f2ca2a82000
mmap(0x7f2ca2a88000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f2ca2a88000
close(3)                                = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f2ca2c90000
arch_prctl(ARCH_SET_FS, 0x7f2ca2c90700) = 0
mprotect(0x7f2ca2a82000, 16384, PROT_READ) = 0
mprotect(0x55d596384000, 4096, PROT_READ) = 0
mprotect(0x7f2ca2caf000, 4096, PROT_READ) = 0
munmap(0x7f2ca2c92000, 103509)          = 0
read(0, "\30\0012s\0015p\0015v\0011x\0010b\0010o\0015f\n", 1) = 23
--- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, si_addr=NULL} ---
+++ killed by SIGSEGV +++
Segmentation fault


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

diff --git a/drivers/staging/speakup/speakup_soft.c b/drivers/staging/speakup/speakup_soft.c
index a61bc41b82d7..f9b405bd052d 100644
--- a/drivers/staging/speakup/speakup_soft.c
+++ b/drivers/staging/speakup/speakup_soft.c
@@ -228,7 +228,7 @@ 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) {
 		if (speakup_info.flushing) {
 			speakup_info.flushing = 0;
 			ch = '\x18';
@@ -257,6 +257,8 @@ static ssize_t softsynthx_read(struct file *fp, char __user *buf, size_t count,
 				0x80 | (ch & 0x3f)
 			};
 
+			if (chars_sent + 2 > count)
+				break;
 			if (copy_to_user(cp, s, sizeof(s)))
 				return -EFAULT;
 
@@ -269,6 +271,8 @@ static ssize_t softsynthx_read(struct file *fp, char __user *buf, size_t count,
 				0x80 | (ch & 0x3f)
 			};
 
+			if (chars_sent + 3 > count)
+				break;
 			if (copy_to_user(cp, s, sizeof(s)))
 				return -EFAULT;
 
-- 
2.18.0.203.gfac676dfb9-goog