coredump masking: reimplementation of dumpable using two flags

This patch changes mm_struct.dumpable to a pair of bit flags.

set_dumpable() converts three-value dumpable to two flags and stores it into
lower two bits of mm_struct.flags instead of mm_struct.dumpable.
get_dumpable() behaves in the opposite way.

[akpm@linux-foundation.org: export set_dumpable]
Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: David Howells <dhowells@redhat.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 56 insertions, 6 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 498f2b3dca20..7bdea7937ee8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1058,9 +1058,9 @@ int flush_old_exec(struct linux_binprm * bprm)
         current->sas_ss_sp = current->sas_ss_size = 0;
 
         if (current->euid == current->uid && current->egid == current->gid)
-                current->mm->dumpable = 1;
+                set_dumpable(current->mm, 1);
         else
-                current->mm->dumpable = suid_dumpable;
+                set_dumpable(current->mm, suid_dumpable);
 
         name = bprm->filename;
 
@@ -1088,7 +1088,7 @@ int flush_old_exec(struct linux_binprm * bprm)
             file_permission(bprm->file, MAY_READ) ||
             (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP)) {
                 suid_keys(current);
-                current->mm->dumpable = suid_dumpable;
+                set_dumpable(current->mm, suid_dumpable);
         }
 
         /* An exec changes our domain. We are no longer part of the thread
@@ -1665,6 +1665,56 @@ fail:
         return core_waiters;
 }
 
+/*
+ * set_dumpable converts traditional three-value dumpable to two flags and
+ * stores them into mm->flags.  It modifies lower two bits of mm->flags, but
+ * these bits are not changed atomically.  So get_dumpable can observe the
+ * intermediate state.  To avoid doing unexpected behavior, get get_dumpable
+ * return either old dumpable or new one by paying attention to the order of
+ * modifying the bits.
+ *
+ * dumpable |   mm->flags (binary)
+ * old  new | initial interim  final
+ * ---------+-----------------------
+ *  0    1  |   00      01      01
+ *  0    2  |   00      10(*)   11
+ *  1    0  |   01      00      00
+ *  1    2  |   01      11      11
+ *  2    0  |   11      10(*)   00
+ *  2    1  |   11      11      01
+ *
+ * (*) get_dumpable regards interim value of 10 as 11.
+ */
+void set_dumpable(struct mm_struct *mm, int value)
+{
+        switch (value) {
+        case 0:
+                clear_bit(MMF_DUMPABLE, &mm->flags);
+                smp_wmb();
+                clear_bit(MMF_DUMP_SECURELY, &mm->flags);
+                break;
+        case 1:
+                set_bit(MMF_DUMPABLE, &mm->flags);
+                smp_wmb();
+                clear_bit(MMF_DUMP_SECURELY, &mm->flags);
+                break;
+        case 2:
+                set_bit(MMF_DUMP_SECURELY, &mm->flags);
+                smp_wmb();
+                set_bit(MMF_DUMPABLE, &mm->flags);
+                break;
+        }
+}
+EXPORT_SYMBOL_GPL(set_dumpable);
+
+int get_dumpable(struct mm_struct *mm)
+{
+        int ret;
+
+        ret = mm->flags & 0x3;
+        return (ret >= 2) ? 2 : ret;
+}
+
 int do_coredump(long signr, int exit_code, struct pt_regs * regs)
 {
         char corename[CORENAME_MAX_SIZE + 1];
@@ -1683,7 +1733,7 @@ int do_coredump(long signr, int exit_code, struct pt_regs * regs)
         if (!binfmt || !binfmt->core_dump)
                 goto fail;
         down_write(&mm->mmap_sem);
-        if (!mm->dumpable) {
+        if (!get_dumpable(mm)) {
                 up_write(&mm->mmap_sem);
                 goto fail;
         }
@@ -1693,11 +1743,11 @@ int do_coredump(long signr, int exit_code, struct pt_regs * regs)
          *      process nor do we know its entire history. We only know it
          *      was tainted so we dump it as root in mode 2.
          */
-        if (mm->dumpable == 2) {        /* Setuid core dump mode */
+        if (get_dumpable(mm) == 2) {    /* Setuid core dump mode */
                 flag = O_EXCL;          /* Stop rewrite attacks */
                 current->fsuid = 0;     /* Dump root private */
         }
-        mm->dumpable = 0;
+        set_dumpable(mm, 0);
 
         retval = coredump_wait(exit_code);
         if (retval < 0)