From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-yb0-x24a.google.com ([2607:f8b0:4002:c09::24a]) by bombadil.infradead.org with esmtps (Exim 4.90_1 #2 (Red Hat Linux)) id 1fbe35-0003kk-8A for linux-mtd@lists.infradead.org; Sat, 07 Jul 2018 03:38:16 +0000 Received: by mail-yb0-x24a.google.com with SMTP id y10-v6so11182610ybj.20 for ; Fri, 06 Jul 2018 20:38:03 -0700 (PDT) MIME-Version: 1.0 Date: Sat, 7 Jul 2018 05:37:22 +0200 Message-Id: <20180707033722.219468-1-jannh@google.com> Subject: [PATCH] mtdchar: fix overflows in adjustment of `count` From: Jann Horn To: David Woodhouse , Brian Norris , Boris Brezillon , Marek Vasut , Richard Weinberger , jannh@google.com Cc: linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , The first checks in mtdchar_read() and mtdchar_write() attempt to limit `count` such that `*ppos + count <= mtd->size`. However, they ignore the possibility of `*ppos > mtd->size`, allowing the calculation of `count` to wrap around. `mtdchar_lseek()` prevents seeking beyond mtd->size, but the pread/pwrite syscalls bypass this. I haven't found any codepath on which this actually causes dangerous behavior, but it seems like a sensible change anyway. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Jann Horn --- drivers/mtd/mtdchar.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c index cd67c85cc87d..02389528f622 100644 --- a/drivers/mtd/mtdchar.c +++ b/drivers/mtd/mtdchar.c @@ -160,8 +160,12 @@ static ssize_t mtdchar_read(struct file *file, char __user *buf, size_t count, pr_debug("MTD_read\n"); - if (*ppos + count > mtd->size) - count = mtd->size - *ppos; + if (*ppos + count > mtd->size) { + if (*ppos < mtd->size) + count = mtd->size - *ppos; + else + count = 0; + } if (!count) return 0; @@ -246,7 +250,7 @@ static ssize_t mtdchar_write(struct file *file, const char __user *buf, size_t c pr_debug("MTD_write\n"); - if (*ppos == mtd->size) + if (*ppos >= mtd->size) return -ENOSPC; if (*ppos + count > mtd->size) -- 2.18.0.203.gfac676dfb9-goog