[patch 42/53] elf core dump: fix get_user use

[Greg KH]

2.6.28-stable review patch.  If anyone has any objections, please let us know.
------------------

From: Roland McGrath <roland@xxxxxxxxxx>

commit 92dc07b1f988e8c237a38e23be660b9b8533e6fd upstream.

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@xxxxxxxxxx>
Signed-off-by: Roland McGrath <roland@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

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

--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1196,9 +1196,11 @@ static unsigned long vma_dump_size(struc
 	 * 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.
 		 */
@@ -1211,7 +1213,15 @@ static unsigned long vma_dump_size(struc
 		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;
 	}
 

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