Messages in this thread | | | Subject | Re: [PATCH] drivers: char: mem: Check {read,write}_kmem() addresses | From | Robin Murphy <> | Date | Tue, 31 May 2016 17:45:14 +0100 |
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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@huawei.com> >> Signed-off-by: Robin Murphy <robin.murphy@arm.com> >> --- >> >> 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 >
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