On 02/08/2014 12:55 PM, Andy Lutomirski wrote: > On Sat, Feb 8, 2014 at 9:15 AM, Richard Yao <ryao@xxxxxxxxxx> wrote: >> On 02/08/2014 01:51 AM, Andy Lutomirski wrote: >>> On Fri, Feb 7, 2014 at 11:55 AM, Dominique Martinet >>> <dominique.martinet@xxxxxx> wrote: >>> Hi, >>>> >>>> Andy Lutomirski wrote on Fri, Feb 07, 2014: >>>>> I can't get modules to load from 9p. The problem seems to be that a call like: >>>>> >>>>> kernel_read(f.file, 0, (char *)(info->hdr),, 115551); >>>>> >>>>> is filling the buffer with mostly zeros (or, more likely, just doing >>>>> nothing at all). The call is in module.c, and the fs is mounted with: >>>>> >>>>> mount -t 9p -o ro,version=9p2000.L,trans=virtio,access=any hostroot /newroot/ >>>>> >>>>> This is really easy to test: grab a copy of virtme >>>>> (https://git.kernel.org/cgit/utils/kernel/virtme/virtme.git/), build >>>>> an appropriate kernel, and run it with virtme-runkernel. Then try to >>>>> insmod any module built for that kernel. It won't work. >>>>> >>>>> Oddly, running executables from the same fs works, and *copying* a >>>>> module to tmpfs and insmoding it there also works. >>>>> >>>>> I'm kind of at a loss debugging this myself. I'd expect that if >>>>> kernel_read were that broken on 9p, then I'd see more obvious >>>>> problems. >>>>> >>>>> This problem exists in at least 3.12 and a recent -linus tree. >>>> >>>> That's been reported a couple of times[1] since two months ago, there's a >>>> fix that might or might or might not make it in the tree (Eric?) there: >>>> http://www.spinics.net/lists/linux-virtualization/msg21716.html >>>> >>>> I'm pretty confident that will do it for you, but would be good to hear >>>> you confirm it again :) >>> >>> That fixes it for me. I think it can't be a module address in >>> finit_module, though -- it's an intermediate vmalloc buffer. It >>> could, however (in principle) be an address in module data, so the >>> full check is probably good. >>> >>> Can one of you send this to Linus and tag it for -stable? I can >>> trigger this bug without getting an OOPS, which means that 9p is >>> overwriting random memory, which puts it in the category of rather bad >>> bugs. I suspect that this is because I don't have >>> CONFIG_DEBUG_VIRTUAL set. >>> >>> (I can't immediately spot any code that would trigger this from user >>> space without being root, so it's probably not a security bug.) >>> >>> --Andy >>> >> >> I have already submitted it for inclusion a couple of times. >> >> The first time was my first time doing any sort of Linux patch >> submission. At the time, I was unaware of ./scripts/get_maintainer.pl >> and sent the patch to only a subset of the correct people. Consequently, >> it was not submitted properly for acceptance by the subsystem maintainer. >> >> The second time was a week ago. I had taken advice from Greg >> Koah-Hartman to use ./scripts/get_maintainer.pl to determine the correct >> recipients. It was initially accepted by the subsystem maintainer and >> then rejected. This patch uses is_vmalloc_or_module_addr(), which is not >> exported for use in kernel modules. Using it causes a build failure when >> CONFIG_NET_9P_VIRTIO=m is set in .config. >> >> I will make a third attempt to mainline this over the next week. Later >> today, I will submit a patch exporting is_vmalloc_or_module_addr(). >> After it has been accepted into mainline, I will resubmit this patch, >> which should then be accepted. This should bring this patch into Linus' >> tree sometime in the next few weeks. > > I would consider asking some mm people (cc'd) how this is supposed to > work -- that is, what the appropriate way of mapping a kernel virtual > address to a struct page is. > > I suspect that the answer might be unpleasant: what happens if the > address is neither in the linear map nor in vmalloc space? For > example, it could be ioremapped. (I have no idea under what useful > conditions the 9pnet code wants to zero-copy a buffer, but I suspect > that there are exactly zero performance-critical users of kernel_read > and kernel_write. Presumably this is for skbs or something.) I > suspect that the right fix is to just fall back to non-zero-copy if > the page is neither vmalloc'd nor linear-mapped, which should be > doable without new exports. > > --Andy > That is only possible if someone calls p9_client_read()/p9_client_write() on an ioremapped address, which is an entirely different problem.
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