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    Subject[PATCH 3.16 233/233] Sanitize 'move_pages()' permission checks
    3.16.48-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Linus Torvalds <torvalds@linux-foundation.org>

    commit 197e7e521384a23b9e585178f3f11c9fa08274b9 upstream.

    The 'move_paghes()' system call was introduced long long ago with the
    same permission checks as for sending a signal (except using
    CAP_SYS_NICE instead of CAP_SYS_KILL for the overriding capability).

    That turns out to not be a great choice - while the system call really
    only moves physical page allocations around (and you need other
    capabilities to do a lot of it), you can check the return value to map
    out some the virtual address choices and defeat ASLR of a binary that
    still shares your uid.

    So change the access checks to the more common 'ptrace_may_access()'
    model instead.

    This tightens the access checks for the uid, and also effectively
    changes the CAP_SYS_NICE check to CAP_SYS_PTRACE, but it's unlikely that
    anybody really _uses_ this legacy system call any more (we hav ebetter
    NUMA placement models these days), so I expect nobody to notice.

    Famous last words.

    Reported-by: Otto Ebeling <otto.ebeling@iki.fi>
    Acked-by: Eric W. Biederman <ebiederm@xmission.com>
    Cc: Willy Tarreau <w@1wt.eu>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    [bwh: Backported to 3.16: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    mm/migrate.c | 11 +++--------
    1 file changed, 3 insertions(+), 8 deletions(-)

    --- a/mm/migrate.c
    +++ b/mm/migrate.c
    @@ -38,6 +38,7 @@
    #include <linux/gfp.h>
    #include <linux/balloon_compaction.h>
    #include <linux/mmu_notifier.h>
    +#include <linux/ptrace.h>

    #include <asm/tlbflush.h>

    @@ -1484,7 +1485,6 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid,
    const int __user *, nodes,
    int __user *, status, int, flags)
    {
    - const struct cred *cred = current_cred(), *tcred;
    struct task_struct *task;
    struct mm_struct *mm;
    int err;
    @@ -1508,14 +1508,9 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid,

    /*
    * Check if this process has the right to modify the specified
    - * process. The right exists if the process has administrative
    - * capabilities, superuser privileges or the same
    - * userid as the target process.
    - */
    - tcred = __task_cred(task);
    - if (!uid_eq(cred->euid, tcred->suid) && !uid_eq(cred->euid, tcred->uid) &&
    - !uid_eq(cred->uid, tcred->suid) && !uid_eq(cred->uid, tcred->uid) &&
    - !capable(CAP_SYS_NICE)) {
    + * process. Use the regular "ptrace_may_access()" checks.
    + */
    + if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
    rcu_read_unlock();
    err = -EPERM;
    goto out;
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