Re: [PATCH] kmsg: limit message size
From: Sasha Levin
Date: Thu May 10 2012 - 12:08:55 EST
On Wed, May 9, 2012 at 12:57 AM, Andrew Morton
> On Sat, 5 May 2012 16:56:12 +0200
> Sasha Levin <levinsasha928@xxxxxxxxx> wrote:
>> There are no size checks in kmsg_write(), and we try allocating enough
>> memory to store everything userspace gave us, which may be too much for
>> kmalloc to allocate.
>> Furthermore, we can have an integer overflow if len==INT_MAX, in that case
>> we'll corrupt kernel memory.
>> This was tested with several userspace programs that write to kmsg, and haven't
>> found a case where the program attempts to write more than PAGE_SIZE.
>> Signed-off-by: Sasha Levin <levinsasha928@xxxxxxxxx>
>> drivers/char/mem.c | 3 +++
>> 1 files changed, 3 insertions(+), 0 deletions(-)
>> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
>> index d6e9d08..c90964b 100644
>> --- a/drivers/char/mem.c
>> +++ b/drivers/char/mem.c
>> @@ -815,6 +815,9 @@ static ssize_t kmsg_writev(struct kiocb *iocb, const struct iovec *iv,
>> ssize_t ret = -EFAULT;
>> size_t len = iov_length(iv, count);
>> + if (len > PAGE_SIZE)
>> + return -E2BIG;
>> line = kmalloc(len + 1, GFP_KERNEL);
>> if (line == NULL)
>> return -ENOMEM;
> Well, this is a write(), and write() is permitted to return
> less-than-asked-for. So what we could do here is to write the first N
> bytes and then return N to userspace. Well-behaved userspace will
> notice this and then do some more writing from offset N.
> Could I ask that you try this and test it a bit, see whether there is
> any well-behaved userspace out there? Things like cat and echo
> _should_ dtrt.
Yup, cat and echo do dtrt. I'll resend a patch which limits the write
instead of canceling it altogether.
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