elf core dump: fix get_user use

The elf_core_dump() code does its work with set_fs(KERNEL_DS) in force,
so vma_dump_size() needs to switch back with set_fs(USER_DS) to safely
use get_user() for a normal user-space address.

Checking for VM_READ optimizes out the case where get_user() would fail
anyway.  The vm_file check here was already superfluous given the control
flow earlier in the function, so that is a cleanup/optimization unrelated
to other changes but an obvious and trivial one.

Reported-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Roland McGrath <roland@redhat.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index e3ff2b9e602f..33b7235f853b 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1208,9 +1208,11 @@ static unsigned long vma_dump_size(struct vm_area_struct *vma,
          * check for an ELF header.  If we find one, dump the first page to
          * aid in determining what was mapped here.
          */
-        if (FILTER(ELF_HEADERS) && vma->vm_file != NULL && vma->vm_pgoff == 0) {
+        if (FILTER(ELF_HEADERS) &&
+            vma->vm_pgoff == 0 && (vma->vm_flags & VM_READ)) {
                 u32 __user *header = (u32 __user *) vma->vm_start;
                 u32 word;
+                mm_segment_t fs = get_fs();
                 /*
                  * Doing it this way gets the constant folded by GCC.
                  */
@@ -1223,7 +1225,15 @@ static unsigned long vma_dump_size(struct vm_area_struct *vma,
                 magic.elfmag[EI_MAG1] = ELFMAG1;
                 magic.elfmag[EI_MAG2] = ELFMAG2;
                 magic.elfmag[EI_MAG3] = ELFMAG3;
-                if (get_user(word, header) == 0 && word == magic.cmp)
+                /*
+                 * Switch to the user "segment" for get_user(),
+                 * then put back what elf_core_dump() had in place.
+                 */
+                set_fs(USER_DS);
+                if (unlikely(get_user(word, header)))
+                        word = 0;
+                set_fs(fs);
+                if (word == magic.cmp)
                         return PAGE_SIZE;
         }