[PATCH] elf core dump: fix get_user use

[Roland McGrath <roland@redhat.com>]

The following changes since commit 6cec50838ed04a9833fb5549f698d3756bbe7e72:
  Linus Torvalds (1):
        Merge branch 'for-linus' of git://git.kernel.org/.../tiwai/sound-2.6

are available in the git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/frob/linux-2.6-roland.git to-linus

Roland McGrath (1):
      elf core dump: fix get_user use

 fs/binfmt_elf.c |   14 ++++++++++++--
 1 files changed, 12 insertions(+), 2 deletions(-)

Thanks,
Roland

---
[PATCH] 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>
---
 fs/binfmt_elf.c |   14 ++++++++++++--
 1 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index e3ff2b9..33b7235 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;
 	}