mm: fix up some user-visible effects of the stack guard page

commit d7824370e26325c881b665350ce64fb0a4fde24a upstream.

This commit makes the stack guard page somewhat less visible to user
space. It does this by:

 - not showing the guard page in /proc/<pid>/maps

   It looks like lvm-tools will actually read /proc/self/maps to figure
   out where all its mappings are, and effectively do a specialized
   "mlockall()" in user space.  By not showing the guard page as part of
   the mapping (by just adding PAGE_SIZE to the start for grows-up
   pages), lvm-tools ends up not being aware of it.

 - by also teaching the _real_ mlock() functionality not to try to lock
   the guard page.

   That would just expand the mapping down to create a new guard page,
   so there really is no point in trying to lock it in place.

It would perhaps be nice to show the guard page specially in
/proc/<pid>/maps (or at least mark grow-down segments some way), but
let's not open ourselves up to more breakage by user space from programs
that depends on the exact deails of the 'maps' file.

Special thanks to Henrique de Moraes Holschuh for diving into lvm-tools
source code to see what was going on with the whole new warning.

Reported-and-tested-by: François Valenduc <francois.valenduc@tvcablenet.be
Reported-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 47f5b145f56eee5aaefc3b52aac8fa78b978b692..3544f60b8739a5b5a06206354dad09adc3ad3f4b 100644 (file)
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -210,6 +210,7 @@ static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
        int flags = vma->vm_flags;
        unsigned long ino = 0;
        unsigned long long pgoff = 0;
+       unsigned long start;
        dev_t dev = 0;
        int len;
 
@@ -220,8 +221,13 @@ static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
                pgoff = ((loff_t)vma->vm_pgoff) << PAGE_SHIFT;
        }
 
+       /* We don't show the stack guard page in /proc/maps */
+       start = vma->vm_start;
+       if (vma->vm_flags & VM_GROWSDOWN)
+               start += PAGE_SIZE;
+
        seq_printf(m, "%08lx-%08lx %c%c%c%c %08llx %02x:%02x %lu %n",
-                       vma->vm_start,
+                       start,
                        vma->vm_end,
                        flags & VM_READ ? 'r' : '-',
                        flags & VM_WRITE ? 'w' : '-',

diff --git a/mm/mlock.c b/mm/mlock.c
index 8f4e2dfceec1c88e796a6bafb78e7422f06bdb72..8268859720ff63445d777edc4f7110c36e3e76bb 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -167,6 +167,14 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
        if (vma->vm_flags & VM_WRITE)
                gup_flags |= FOLL_WRITE;
 
+       /* We don't try to access the guard page of a stack vma */
+       if (vma->vm_flags & VM_GROWSDOWN) {
+               if (start == vma->vm_start) {
+                       start += PAGE_SIZE;
+                       nr_pages--;
+               }
+       }
+
        while (nr_pages > 0) {
                int i;