Re: [PATCH] drivers: char: mem: Check {read,write}_kmem() addresses

[Robin Murphy]

On 31/05/16 14:46, Catalin Marinas wrote:
> On Tue, May 31, 2016 at 01:52:45PM +0100, Robin Murphy wrote:
> > Arriving at read_kmem() with an offset representing a bogus kernel
> > address (e.g. 0 from a simple "cat /dev/kmem") leads to copy_to_user
> > faulting on the kernel-space read.
> >
> > x86_64 happens to get away with this since the optimised implementation
> > uses "rep movs*", thus the user write (which is allowed to fault) and
> > the kernel read are the same instruction, the kernel-side fault falls
> > into the userspace fixup handler and a chain of events transpires
> > leading to returning the expected -EFAULT. On other architectures,
> > though, the read is not covered by the fixup entry for the write, and we
> > get a straightforward "Unable to hande kernel paging request..." dump.
> >
> > The more typical use-case of mmap_kmem() already validates the address
> > with pfn_valid() as one might expect, so let's make that consistent
> > across {read,write}_kem() too.
> >
> > Reported-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> > ---
> >
> > I'm not sure if this warrants going to stable or not, as it's really
> > just making an existing failure case more graceful and less confusing.
> >
> >   drivers/char/mem.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> > index 71025c2f6bbb..64c766023b15 100644
> > --- a/drivers/char/mem.c
> > +++ b/drivers/char/mem.c
> > @@ -384,6 +384,9 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
> >   	char *kbuf; /* k-addr because vread() takes vmlist_lock rwlock */
> >   	int err = 0;
> >
> > +	if (!pfn_valid(PFN_DOWN(p)))
> > +		return -EFAULT;
> > +
> >   	read = 0;
> >   	if (p < (unsigned long) high_memory) {
> >   		low_count = count;
> > @@ -512,6 +515,9 @@ static ssize_t write_kmem(struct file *file, const char __user *buf,
> >   	char *kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */
> >   	int err = 0;
> >
> > +	if (!pfn_valid(PFN_DOWN(p)))
> > +		return -EFAULT;
>
> Since the /dev/kmem interface is about kernel virtual address rather
> than physical (like /dev/mem), the pfn may not always be mapped. I think
> a better check would be to use kern_addr_valid(kaddr) just before
> copy_(to|from)_user (a similar approach is taken by read_kcore()). The
> downside is that it breaks a couple of configurations where
> kern_addr_valid() is 0:

Well, the mmap() case, which is arguably the "normal" access method, 
looks to have been enforcing pfn_valid since pretty much forever[1] so I 
struggle to imagine how much anyone will actually care. In my view it's 
more just that "do a silly thing and get an error" seems preferable to 
"do a silly thing and get a scary backtrace".

Robin.

[1]:http://lwn.net/Articles/147901/ - I particularly enjoyed 
"[...]chances are that /dev/kmem will not survive into 2.6.14"

> - x86_32 with !CONFIG_FLATMEM
> - alpha with CONFIG_DISCONTIGMEM