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Message-Id: <6718ede2-1fcb-1a8f-a116-250eef6416c7@linux.vnet.ibm.com> Date: Tue, 23 Apr 2019 13:16:42 +0200 From: Laurent Dufour <ldufour@...ux.vnet.ibm.com> To: Michael Ellerman <mpe@...erman.id.au>, Thomas Gleixner <tglx@...utronix.de>, Dave Hansen <dave.hansen@...ux.intel.com> Cc: LKML <linux-kernel@...r.kernel.org>, rguenther@...e.de, mhocko@...e.com, vbabka@...e.cz, luto@...capital.net, x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, stable@...r.kernel.org Subject: Re: [PATCH] x86/mpx: fix recursive munmap() corruption Le 20/04/2019 à 12:31, Michael Ellerman a écrit : > Thomas Gleixner <tglx@...utronix.de> writes: >> On Mon, 1 Apr 2019, Dave Hansen wrote: >>> diff -puN mm/mmap.c~mpx-rss-pass-no-vma mm/mmap.c >>> --- a/mm/mmap.c~mpx-rss-pass-no-vma 2019-04-01 06:56:53.409411123 -0700 >>> +++ b/mm/mmap.c 2019-04-01 06:56:53.423411123 -0700 >>> @@ -2731,9 +2731,17 @@ int __do_munmap(struct mm_struct *mm, un >>> return -EINVAL; >>> >>> len = PAGE_ALIGN(len); >>> + end = start + len; >>> if (len == 0) >>> return -EINVAL; >>> >>> + /* >>> + * arch_unmap() might do unmaps itself. It must be called >>> + * and finish any rbtree manipulation before this code >>> + * runs and also starts to manipulate the rbtree. >>> + */ >>> + arch_unmap(mm, start, end); >> >> ... >> >>> -static inline void arch_unmap(struct mm_struct *mm, struct vm_area_struct *vma, >>> - unsigned long start, unsigned long end) >>> +static inline void arch_unmap(struct mm_struct *mm, unsigned long start, >>> + unsigned long end) >> >> While you fixed up the asm-generic thing, this breaks arch/um and >> arch/unicorn32. For those the fixup is trivial by removing the vma >> argument. >> >> But itt also breaks powerpc and there I'm not sure whether moving >> arch_unmap() to the beginning of __do_munmap() is safe. Micheal??? > > I don't know for sure but I think it should be fine. That code is just > there to handle CRIU unmapping/remapping the VDSO. So that either needs > to happen while the process is stopped or it needs to handle races > anyway, so I don't see how the placement within the unmap path should > matter. My only concern is the error path. Calling arch_unmap() before handling any error case means that it will have to be undo and there is no way to do so. I don't know what is the rational to move arch_unmap() to the beginning of __do_munmap() but the error paths must be managed. >> Aside of that the powerpc variant looks suspicious: >> >> static inline void arch_unmap(struct mm_struct *mm, >> unsigned long start, unsigned long end) >> { >> if (start <= mm->context.vdso_base && mm->context.vdso_base < end) >> mm->context.vdso_base = 0; >> } >> >> Shouldn't that be: >> >> if (start >= mm->context.vdso_base && mm->context.vdso_base < end) >> >> Hmm? > > Yeah looks pretty suspicious. I'll follow-up with Laurent who wrote it. > Thanks for spotting it! I've to admit that I had to read that code carefully before answering. There are 2 assumptions here: 1. 'start' and 'end' are page aligned (this is guaranteed by __do_munmap(). 2. the VDSO is 1 page (this is guaranteed by the union vdso_data_store on powerpc). The idea is to handle a munmap() call surrounding the VDSO area: | VDSO | ^start ^end This is covered by this test, as the munmap() matching the exact boundaries of the VDSO is handled too. Am I missing something ? Cheers, Laurent.
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