Re: /dev/kmem BUG on mmap

[Johannes Weiner]

On Mon, Jul 30, 2012 at 12:28:35AM +0200, Sasha Levin wrote:
> Hi all,
> 
> I was poking around /dev/kmem related code, and noticed the following in mmap_kmem():
> 
>         /* Turn a kernel-virtual address into a physical page frame */
>         pfn = __pa((u64)vma->vm_pgoff << PAGE_SHIFT) >> PAGE_SHIFT;
> 
> Which looked odd since vm_pgoff is the offset into the mapping, so
> I'd assume that PAGE_OFFSET should be added to it as well, otherwise
> we get an invalid address.

It's supposed to be used with kernel offsets in the first place,
i.e. vma->vm_pgoff << PAGE_SHIFT should actually be a kernel virtual
address.  See 6d3154c Revert "[PATCH] Fix up mmap_kmem".

> I tested it by writing something like this:
> 
> 	int main(void)
> 	{
> 	        int fd;
> 	        void *addr;
> 	
> 	        fd = open("/dev/kmem", O_RDONLY);
> 	        addr = mmap(NULL, 4096, PROT_READ, MAP_PRIVATE, fd, 4096);
> 	
> 	        return 0;
> 	}
> 
> Which indeed triggered a VM_BUG:
> 
> [   32.285431] kernel BUG at arch/x86/mm/physaddr.c:18!

x86's debug-version of __pa() triggers that bug.  I'm reluctant to add
a whole lot of error checking to this interface, given that you should
already know what you are doing.  OTOH, crashing like this is not very
nice, either.

Is there a portable way to check if an address is a kernel virtual
one?  It looks like comparing to PAGE_OFFSET would work on most archs,
but not necessarily on powerpc for example.

Johannes

> [   32.285431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> [   32.285431] CPU 0
> [   32.285431] Pid: 5643, comm: a.out Tainted: G        W    3.5.0-next-20120727-sasha #504
> [   32.285431] RIP: 0010:[<ffffffff810acd97>]  [<ffffffff810acd97>] __phys_addr+0x57/0xa0
> [   32.285431] RSP: 0018:ffff88000be67d68  EFLAGS: 00010213
> [   32.285431] RAX: ffff87ffffffffff RBX: ffff88000d67cb00 RCX: 00000000000080d0
> [   32.285431] RDX: 0000000000000071 RSI: ffff88000bfc8dc8 RDI: 0000000000001000
> [   32.285431] RBP: ffff88000be67d68 R08: 0000000000000001 R09: ffff88000bfc8dc8
> [   32.285431] R10: ffff88000bfc81f8 R11: 0000000000000002 R12: ffff88000bfc8dc8
> [   32.285431] R13: 00007f26f80e6000 R14: ffff88000bf81000 R15: ffff88000bfc8dc8
> [   32.285431] FS:  00007f26f80e8700(0000) GS:ffff88000d800000(0000) knlGS:0000000000000000
> [   32.285431] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   32.285431] CR2: 00007f26f7c07d50 CR3: 000000000bfb8000 CR4: 00000000000406f0
> [   32.285431] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   32.285431] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [   32.285431] Process a.out (pid: 5643, threadinfo ffff88000be66000, task ffff88000bf2b000)
> [   32.285431] Stack:
> [   32.285431]  ffff88000be67d88 ffffffff81bb0737 ffff88000bfc95e0 ffff88000bfc95f0
> [   32.285431]  ffff88000be67e48 ffffffff812163ae ffff88000d67cb00 0000000000000001
> [   32.285431]  0000000000000000 0000000000000000 ffff88000be67dd8 ffff88000bfc81f8
> [   32.285431] Call Trace:
> [   32.285431]  [<ffffffff81bb0737>] mmap_kmem+0x27/0x90
> [   32.285431]  [<ffffffff812163ae>] mmap_region+0x35e/0x5f0
> [   32.285431]  [<ffffffff812168f9>] do_mmap_pgoff+0x2b9/0x350
> [   32.285431]  [<ffffffff8120120c>] ? vm_mmap_pgoff+0x6c/0xb0
> [   32.285431]  [<ffffffff81201224>] vm_mmap_pgoff+0x84/0xb0
> [   32.285431]  [<ffffffff8124f280>] ? fget_raw+0x260/0x260
> [   32.285431]  [<ffffffff81213d9e>] sys_mmap_pgoff+0x15e/0x190
> [   32.285431]  [<ffffffff8198ab2e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> [   32.285431]  [<ffffffff8106e4ed>] sys_mmap+0x1d/0x20
> [   32.285431]  [<ffffffff8361f6f9>] system_call_fastpath+0x16/0x1b
> [   32.285431] Code: 00 00 00 00 eb fe 66 0f 1f 44 00 00 48 03 05 91 02 78 03 eb 57 0f 1f 80 00 00 00 00 48 b8 ff ff ff ff ff 87 ff ff 48 39 c7 77 11 <0f> 0b 0f 1f 80 00 00 00 00 eb fe 66 0f 1f 44 00 00 48 b8 00 00
> [   32.285431] RIP  [<ffffffff810acd97>] __phys_addr+0x57/0xa0
> [   32.285431]  RSP <ffff88000be67d68>
> 
> I could send a patch to do what I think it's supposed to be doing, but I find it odd since apparently /dev/kmem has been broken for a while now - which doesn't make sense.
> 
> What am I missing?
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