| Message ID | 20201207094818.8518-1-osalvador@suse.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm,memory_failure: Always pin the page in madvise_inject_error | expand |
On Mon, 7 Dec 2020 10:48:18 +0100 Oscar Salvador <osalvador@suse.de> wrote: > madvise_inject_error() uses get_user_pages_fast to translate the > address we specified to a page. > After [1], we drop the extra reference count for memory_failure() path. > That commit says that memory_failure wanted to keep the pin in order > to take the page out of circulation. > > The truth is that we need to keep the page pinned, otherwise the > page might be re-used after the put_page() and we can end up messing > with someone else's memory. > > E.g: > > CPU0 > process X CPU1 > madvise_inject_error > get_user_pages > put_page > page gets reclaimed > process Y allocates the page > memory_failure > // We mess with process Y memory > > madvise() is meant to operate on a self address space, so messing with > pages that do not belong to us seems the wrong thing to do. > To avoid that, let us keep the page pinned for memory_failure as well. > > Pages for DAX mappings will release this extra refcount in > memory_failure_dev_pagemap. Does the bug have any known user-visible effects? Is a deliberate exploit conceivable? IOW, cc:stable and if so, why?
On Mon, Dec 07, 2020 at 06:22:00PM -0800, Andrew Morton wrote: > On Mon, 7 Dec 2020 10:48:18 +0100 Oscar Salvador <osalvador@suse.de> wrote: > > > madvise_inject_error() uses get_user_pages_fast to translate the > > address we specified to a page. > > After [1], we drop the extra reference count for memory_failure() path. > > That commit says that memory_failure wanted to keep the pin in order > > to take the page out of circulation. > > > > The truth is that we need to keep the page pinned, otherwise the > > page might be re-used after the put_page() and we can end up messing > > with someone else's memory. > > > > E.g: > > > > CPU0 > > process X CPU1 > > madvise_inject_error > > get_user_pages > > put_page > > page gets reclaimed > > process Y allocates the page > > memory_failure > > // We mess with process Y memory > > > > madvise() is meant to operate on a self address space, so messing with > > pages that do not belong to us seems the wrong thing to do. > > To avoid that, let us keep the page pinned for memory_failure as well. > > > > Pages for DAX mappings will release this extra refcount in > > memory_failure_dev_pagemap. > > Does the bug have any known user-visible effects? Is a deliberate > exploit conceivable? > > IOW, cc:stable and if so, why? This interface is a testing feature and only available only for privileged (CAP_SYS_ADMIN) users, so I don't think that this bug is critical. But if someone think it need to go to stable, I'm fine with that. Thanks, Naoya Horiguchi
On Mon, Dec 07, 2020 at 10:48:18AM +0100, Oscar Salvador wrote: > madvise_inject_error() uses get_user_pages_fast to translate the > address we specified to a page. > After [1], we drop the extra reference count for memory_failure() path. > That commit says that memory_failure wanted to keep the pin in order > to take the page out of circulation. > > The truth is that we need to keep the page pinned, otherwise the > page might be re-used after the put_page() and we can end up messing > with someone else's memory. > > E.g: > > CPU0 > process X CPU1 > madvise_inject_error > get_user_pages > put_page > page gets reclaimed > process Y allocates the page > memory_failure > // We mess with process Y memory > > madvise() is meant to operate on a self address space, so messing with > pages that do not belong to us seems the wrong thing to do. > To avoid that, let us keep the page pinned for memory_failure as well. > > Pages for DAX mappings will release this extra refcount in > memory_failure_dev_pagemap. > > [1] ("23e7b5c2e271: mm, madvise_inject_error: > Let memory_failure() optionally take a page reference") > > Signed-off-by: Oscar Salvador <osalvador@suse.de> > Suggested-by: Vlastimil Babka <vbabka@suse.cz> > Fixes: 23e7b5c2e271 ("mm, madvise_inject_error: Let memory_failure() optionally take a page reference") Thank you very much. Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
diff --git a/mm/madvise.c b/mm/madvise.c index c6b5524add58..19edddba196d 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -907,14 +907,7 @@ static int madvise_inject_error(int behavior, } else { pr_info("Injecting memory failure for pfn %#lx at process virtual address %#lx\n", pfn, start); - /* - * Drop the page reference taken by get_user_pages_fast(). In - * the absence of MF_COUNT_INCREASED the memory_failure() - * routine is responsible for pinning the page to prevent it - * from being released back to the page allocator. - */ - put_page(page); - ret = memory_failure(pfn, 0); + ret = memory_failure(pfn, MF_COUNT_INCREASED); } if (ret) diff --git a/mm/memory-failure.c b/mm/memory-failure.c index 869ece2a1de2..064d605cc416 100644 --- a/mm/memory-failure.c +++ b/mm/memory-failure.c @@ -1258,6 +1258,12 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags, loff_t start; dax_entry_t cookie; + if (flags & MF_COUNT_INCREASED) + /* + * Drop the extra refcount in case we come from madvise(). + */ + put_page(page); + /* * Prevent the inode from being freed while we are interrogating * the address_space, typically this would be handled by
madvise_inject_error() uses get_user_pages_fast to translate the address we specified to a page. After [1], we drop the extra reference count for memory_failure() path. That commit says that memory_failure wanted to keep the pin in order to take the page out of circulation. The truth is that we need to keep the page pinned, otherwise the page might be re-used after the put_page() and we can end up messing with someone else's memory. E.g: CPU0 process X CPU1 madvise_inject_error get_user_pages put_page page gets reclaimed process Y allocates the page memory_failure // We mess with process Y memory madvise() is meant to operate on a self address space, so messing with pages that do not belong to us seems the wrong thing to do. To avoid that, let us keep the page pinned for memory_failure as well. Pages for DAX mappings will release this extra refcount in memory_failure_dev_pagemap. [1] ("23e7b5c2e271: mm, madvise_inject_error: Let memory_failure() optionally take a page reference") Signed-off-by: Oscar Salvador <osalvador@suse.de> Suggested-by: Vlastimil Babka <vbabka@suse.cz> Fixes: 23e7b5c2e271 ("mm, madvise_inject_error: Let memory_failure() optionally take a page reference") --- mm/madvise.c | 9 +-------- mm/memory-failure.c | 6 ++++++ 2 files changed, 7 insertions(+), 8 deletions(-)