lkml.org 
[lkml]   [2017]   [Jun]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH 3.10 248/250] mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for thp
    Date
    From: Keno Fischer <keno@juliacomputing.com>

    commit 8310d48b125d19fcd9521d83b8293e63eb1646aa upstream.

    In commit 19be0eaffa3a ("mm: remove gup_flags FOLL_WRITE games from
    __get_user_pages()"), the mm code was changed from unsetting FOLL_WRITE
    after a COW was resolved to setting the (newly introduced) FOLL_COW
    instead. Simultaneously, the check in gup.c was updated to still allow
    writes with FOLL_FORCE set if FOLL_COW had also been set.

    However, a similar check in huge_memory.c was forgotten. As a result,
    remote memory writes to ro regions of memory backed by transparent huge
    pages cause an infinite loop in the kernel (handle_mm_fault sets
    FOLL_COW and returns 0 causing a retry, but follow_trans_huge_pmd bails
    out immidiately because `(flags & FOLL_WRITE) && !pmd_write(*pmd)` is
    true.

    While in this state the process is stil SIGKILLable, but little else
    works (e.g. no ptrace attach, no other signals). This is easily
    reproduced with the following code (assuming thp are set to always):

    #include <assert.h>
    #include <fcntl.h>
    #include <stdint.h>
    #include <stdio.h>
    #include <string.h>
    #include <sys/mman.h>
    #include <sys/stat.h>
    #include <sys/types.h>
    #include <sys/wait.h>
    #include <unistd.h>

    #define TEST_SIZE 5 * 1024 * 1024

    int main(void) {
    int status;
    pid_t child;
    int fd = open("/proc/self/mem", O_RDWR);
    void *addr = mmap(NULL, TEST_SIZE, PROT_READ,
    MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
    assert(addr != MAP_FAILED);
    pid_t parent_pid = getpid();
    if ((child = fork()) == 0) {
    void *addr2 = mmap(NULL, TEST_SIZE, PROT_READ | PROT_WRITE,
    MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
    assert(addr2 != MAP_FAILED);
    memset(addr2, 'a', TEST_SIZE);
    pwrite(fd, addr2, TEST_SIZE, (uintptr_t)addr);
    return 0;
    }
    assert(child == waitpid(child, &status, 0));
    assert(WIFEXITED(status) && WEXITSTATUS(status) == 0);
    return 0;
    }

    Fix this by updating follow_trans_huge_pmd in huge_memory.c analogously
    to the update in gup.c in the original commit. The same pattern exists
    in follow_devmap_pmd. However, we should not be able to reach that
    check with FOLL_COW set, so add WARN_ONCE to make sure we notice if we
    ever do.

    [akpm@linux-foundation.org: coding-style fixes]
    Link: http://lkml.kernel.org/r/20170106015025.GA38411@juliacomputing.com
    Signed-off-by: Keno Fischer <keno@juliacomputing.com>
    Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Greg Thelen <gthelen@google.com>
    Cc: Nicholas Piggin <npiggin@gmail.com>
    Cc: Willy Tarreau <w@1wt.eu>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Hugh Dickins <hughd@google.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    [bwh: Backported to 3.2:
    - Drop change to follow_devmap_pmd()
    - pmd_dirty() is not available; check the page flags as in
    can_follow_write_pte()
    - Adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    [mhocko:
    This has been forward ported from the 3.2 stable tree.
    And fixed to return NULL.]
    Reviewed-by: Michal Hocko <mhocko@suse.cz>
    Signed-off-by: Jiri Slaby <jslaby@suse.cz>
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    ---
    mm/huge_memory.c | 19 ++++++++++++++++---
    1 file changed, 16 insertions(+), 3 deletions(-)

    diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    index d21c9ef..3877483 100644
    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -1235,6 +1235,18 @@ out_unlock:
    return ret;
    }

    +/*
    + * foll_force can write to even unwritable pmd's, but only
    + * after we've gone through a cow cycle and they are dirty.
    + */
    +static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page,
    + unsigned int flags)
    +{
    + return pmd_write(pmd) ||
    + ((flags & FOLL_FORCE) && (flags & FOLL_COW) &&
    + page && PageAnon(page));
    +}
    +
    struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
    unsigned long addr,
    pmd_t *pmd,
    @@ -1245,15 +1257,16 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,

    assert_spin_locked(&mm->page_table_lock);

    - if (flags & FOLL_WRITE && !pmd_write(*pmd))
    - goto out;
    -
    /* Avoid dumping huge zero page */
    if ((flags & FOLL_DUMP) && is_huge_zero_pmd(*pmd))
    return ERR_PTR(-EFAULT);

    page = pmd_page(*pmd);
    VM_BUG_ON(!PageHead(page));
    +
    + if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, page, flags))
    + return NULL;
    +
    if (flags & FOLL_TOUCH) {
    pmd_t _pmd;
    /*
    --
    2.8.0.rc2.1.gbe9624a
    \
     
     \ /
      Last update: 2017-06-12 01:17    [W:4.060 / U:0.128 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site