On Tue, Oct 12, 2021 at 10:42:40AM +0200, Greg KH wrote: > On Tue, Oct 12, 2021 at 10:14:27AM +0200, Vlastimil Babka wrote: > > On 10/12/21 10:06, Jan Kara wrote: > > > On Mon 11-10-21 18:52:44, Suren Baghdasaryan wrote: > > >> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > >> > > >> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > >> > > >> commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream. > > >> > > >> Doing a "get_user_pages()" on a copy-on-write page for reading can be > > >> ambiguous: the page can be COW'ed at any time afterwards, and the > > >> direction of a COW event isn't defined. > > >> > > >> Yes, whoever writes to it will generally do the COW, but if the thread > > >> that did the get_user_pages() unmapped the page before the write (and > > >> that could happen due to memory pressure in addition to any outright > > >> action), the writer could also just take over the old page instead. > > >> > > >> End result: the get_user_pages() call might result in a page pointer > > >> that is no longer associated with the original VM, and is associated > > >> with - and controlled by - another VM having taken it over instead. > > >> > > >> So when doing a get_user_pages() on a COW mapping, the only really safe > > >> thing to do would be to break the COW when getting the page, even when > > >> only getting it for reading. > > >> > > >> At the same time, some users simply don't even care. > > >> > > >> For example, the perf code wants to look up the page not because it > > >> cares about the page, but because the code simply wants to look up the > > >> physical address of the access for informational purposes, and doesn't > > >> really care about races when a page might be unmapped and remapped > > >> elsewhere. > > >> > > >> This adds logic to force a COW event by setting FOLL_WRITE on any > > >> copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page > > >> pointer as a result. > > >> > > >> The current semantics end up being: > > >> > > >> - __get_user_pages_fast(): no change. If you don't ask for a write, > > >> you won't break COW. You'd better know what you're doing. > > >> > > >> - get_user_pages_fast(): the fast-case "look it up in the page tables > > >> without anything getting mmap_sem" now refuses to follow a read-only > > >> page, since it might need COW breaking. Which happens in the slow > > >> path - the fast path doesn't know if the memory might be COW or not. > > >> > > >> - get_user_pages() (including the slow-path fallback for gup_fast()): > > >> for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with > > >> very similar semantics to FOLL_FORCE. > > >> > > >> If it turns out that we want finer granularity (ie "only break COW when > > >> it might actually matter" - things like the zero page are special and > > >> don't need to be broken) we might need to push these semantics deeper > > >> into the lookup fault path. So if people care enough, it's possible > > >> that we might end up adding a new internal FOLL_BREAK_COW flag to go > > >> with the internal FOLL_COW flag we already have for tracking "I had a > > >> COW". > > >> > > >> Alternatively, if it turns out that different callers might want to > > >> explicitly control the forced COW break behavior, we might even want to > > >> make such a flag visible to the users of get_user_pages() instead of > > >> using the above default semantics. > > >> > > >> But for now, this is mostly commentary on the issue (this commit message > > >> being a lot bigger than the patch, and that patch in turn is almost all > > >> comments), with that minimal "enable COW breaking early" logic using the > > >> existing FOLL_WRITE behavior. > > >> > > >> [ It might be worth noting that we've always had this ambiguity, and it > > >> could arguably be seen as a user-space issue. > > >> > > >> You only get private COW mappings that could break either way in > > >> situations where user space is doing cooperative things (ie fork() > > >> before an execve() etc), but it _is_ surprising and very subtle, and > > >> fork() is supposed to give you independent address spaces. > > >> > > >> So let's treat this as a kernel issue and make the semantics of > > >> get_user_pages() easier to understand. Note that obviously a true > > >> shared mapping will still get a page that can change under us, so this > > >> does _not_ mean that get_user_pages() somehow returns any "stable" > > >> page ] > > >> > > >> [surenb: backport notes > > >> Since gup_pgd_range does not exist, made appropriate changes on > > >> the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead. > > >> Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd, > > >> gup_huge_pd and gup_pud_range. > > >> Removed FOLL_PIN usage in should_force_cow_break since it's missing in > > >> the earlier kernels.] > > > > > > I'd be really careful with backporting this to stable. There was a lot of > > > userspace breakage caused by this change if I remember right which needed > > > to be fixed up later. There is a nice summary at > > > https://lwn.net/Articles/849638/ and https://lwn.net/Articles/849876/ and > > > some problems are still being found... > > > > Yeah that was my initial reaction. But looks like back in April we agreed > > that backporting only this commit could be feasible - the relevant subthread > > starts around here [1]. The known breakage for just this commit was uffd > > functionality introduced only in 5.7, and strace on dax on pmem (that was > > never properly root caused). 5.4 stable already has the backport since year > > ago, Suren posted 4.14 and 4.19 in April after [1]. Looks like nobody > > reported issues? Continuing with 4.4 and 4.9 makes this consistent at least, > > although the risk of breaking something is always there and the CVE probably > > not worth it, but whatever... > > I have had people "complain" that the issue was not fixed on these older > kernels, now if that is just because those groups have a "it has a CVE > so it must be fixed!" policy or not, it is hard to tell. > > But this seems to be exploitable, and we have a reproducer somewhere > around here, so it would be nice to get resolved for the reason of it > being a bug that we should fix if possible. > > So I would err on the side of "lets merge this" as fixing a known issue > is ALWAYS better than the fear of "maybe something might break". We can > always revert if the latter happens in testing. > > thanks, > > greg k-h When we backported this to the Ubuntu kernel based on 4.4, we found a regression that required commit 38e088546522e1e86d2b8f401a1354ad3a9b3303 ("mm: check VMA flags to avoid invalid PROT_NONE NUMA balancing") as a fix. I tested that this was also the case with the 4.4.y stable-rc tree and I am providing our backport below, which I also tested. The reproducer that regresses reads from /proc/self/mem. Writing to /proc/self/mem seems to have been a bug on 4.4 for a while and is also fixed by this backport, so should be considered in any case. Cascardo.
>From 757597f803f5770ebd7c70d4bf0068aa525c033a Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes <lstoakes@xxxxxxxxx> Date: Mon, 5 Apr 2021 22:36:00 +0200 Subject: [PATCH 4.4] mm: check VMA flags to avoid invalid PROT_NONE NUMA balancing commit 38e088546522e1e86d2b8f401a1354ad3a9b3303 upstream. The NUMA balancing logic uses an arch-specific PROT_NONE page table flag defined by pte_protnone() or pmd_protnone() to mark PTEs or huge page PMDs respectively as requiring balancing upon a subsequent page fault. User-defined PROT_NONE memory regions which also have this flag set will not normally invoke the NUMA balancing code as do_page_fault() will send a segfault to the process before handle_mm_fault() is even called. However if access_remote_vm() is invoked to access a PROT_NONE region of memory, handle_mm_fault() is called via faultin_page() and __get_user_pages() without any access checks being performed, meaning the NUMA balancing logic is incorrectly invoked on a non-NUMA memory region. A simple means of triggering this problem is to access PROT_NONE mmap'd memory using /proc/self/mem which reliably results in the NUMA handling functions being invoked when CONFIG_NUMA_BALANCING is set. This issue was reported in bugzilla (issue 99101) which includes some simple repro code. There are BUG_ON() checks in do_numa_page() and do_huge_pmd_numa_page() added at commit c0e7cad to avoid accidentally provoking strange behaviour by attempting to apply NUMA balancing to pages that are in fact PROT_NONE. The BUG_ON()'s are consistently triggered by the repro. This patch moves the PROT_NONE check into mm/memory.c rather than invoking BUG_ON() as faulting in these pages via faultin_page() is a valid reason for reaching the NUMA check with the PROT_NONE page table flag set and is therefore not always a bug. Link: https://bugzilla.kernel.org/show_bug.cgi?id=99101 Reported-by: Trevor Saunders <tbsaunde@xxxxxxxxxxxx> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> Acked-by: Rik van Riel <riel@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Tim Gardner <tim.gardner@xxxxxxxxxxxxx> Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri@xxxxxxxxxxxxx> [cascardo: context adjustments were necessary] Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx> --- mm/huge_memory.c | 3 --- mm/memory.c | 12 +++++++----- 2 files changed, 7 insertions(+), 8 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index fae45c56e2ee..2f53786098c5 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1340,9 +1340,6 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma, bool was_writable; int flags = 0; - /* A PROT_NONE fault should not end up here */ - BUG_ON(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))); - ptl = pmd_lock(mm, pmdp); if (unlikely(!pmd_same(pmd, *pmdp))) goto out_unlock; diff --git a/mm/memory.c b/mm/memory.c index 360d28224a8e..6bfc6a021c4f 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3209,9 +3209,6 @@ static int do_numa_page(struct mm_struct *mm, struct vm_area_struct *vma, bool was_writable = pte_write(pte); int flags = 0; - /* A PROT_NONE fault should not end up here */ - BUG_ON(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))); - /* * The "pte" at this point cannot be used safely without * validation through pte_unmap_same(). It's of NUMA type but @@ -3304,6 +3301,11 @@ static int wp_huge_pmd(struct mm_struct *mm, struct vm_area_struct *vma, return VM_FAULT_FALLBACK; } +static inline bool vma_is_accessible(struct vm_area_struct *vma) +{ + return vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE); +} + /* * These routines also need to handle stuff like marking pages dirty * and/or accessed for architectures that don't do it in hardware (most @@ -3350,7 +3352,7 @@ static int handle_pte_fault(struct mm_struct *mm, pte, pmd, flags, entry); } - if (pte_protnone(entry)) + if (pte_protnone(entry) && vma_is_accessible(vma)) return do_numa_page(mm, vma, address, entry, pte, pmd); ptl = pte_lockptr(mm, pmd); @@ -3425,7 +3427,7 @@ static int __handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma, if (pmd_trans_splitting(orig_pmd)) return 0; - if (pmd_protnone(orig_pmd)) + if (pmd_protnone(orig_pmd) && vma_is_accessible(vma)) return do_huge_pmd_numa_page(mm, vma, address, orig_pmd, pmd); -- 2.30.2
