CVE-2017-18208: Loop with Unreachable Exit Condition ('Infinite Loop') (CWE-835)
value: The madvise_willneed function in mm/madvise.c in the Linux kernel before 4.14.4 allows local users to cause a denial of service (infinite loop) by triggering use of MADVISE_WILLNEED for a DAX mapping.
CWE description: The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.
CVSS v3 base score: 5.5
Published: 2018-03-01T05:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: mm/madvise.c
Commit date: 2017-11-29 18:40:43 -0800
Commit message: mm/madvise.c: fix madvise() infinite loop under special circumstances

MADVISE_WILLNEED has always been a noop for DAX (formerly XIP) mappings.
Unfortunately madvise_willneed() doesn't communicate this information
properly to the generic madvise syscall implementation.  The calling
convention is quite subtle there.  madvise_vma() is supposed to either
return an error or update &prev otherwise the main loop will never
advance to the next vma and it will keep looping for ever without a way
to get out of the kernel.

It seems this has been broken since introduction.  Nobody has noticed
because nobody seems to be using MADVISE_WILLNEED on these DAX mappings.

[mhocko@suse.com: rewrite changelog]
Link: http://lkml.kernel.org/r/20171127115318.911-1-guoxuenan@huawei.com
Fixes: fe77ba6f4f97 ("[PATCH] xip: madvice/fadvice: execute in place")
Signed-off-by: chenjie <chenjie6@huawei.com>
Signed-off-by: guoxuenan <guoxuenan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: zhangyi (F) <yi.zhang@huawei.com>
Cc: Miao Xie <miaoxie@huawei.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Carsten Otte <cotte@de.ibm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Diff: diff --git a/mm/madvise.c b/mm/madvise.c
index 375cf32087e4a2..751e97aa22106f 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -274,19 +274,18 @@ static long madvise_willneed(struct vm_area_struct *vma,
 			     struct vm_area_struct **prev,
 			     unsigned long start, unsigned long end)
 {
 	struct file *file = vma->vm_file;
 
+	*prev = vma;
 #ifdef CONFIG_SWAP
 	if (!file) {
-		*prev = vma;
 		force_swapin_readahead(vma, start, end);
 		return 0;
 	}
 
 	if (shmem_mapping(file->f_mapping)) {
-		*prev = vma;
 		force_shm_swapin_readahead(vma, start, end,
 					file->f_mapping);
 		return 0;
 	}
 #else
@@ -297,11 +296,10 @@ static long madvise_willneed(struct vm_area_struct *vma,
 	if (IS_DAX(file_inode(file))) {
 		/* no bad return value, but ignore advice */
 		return 0;
 	}
 
-	*prev = vma;
 	start = ((start - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
 	if (end > vma->vm_end)
 		end = vma->vm_end;
 	end = ((end - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
 

Vulnerable code: 		*prev = vma;
		*prev = vma;
	*prev = vma;
Fixed code: 	*prev = vma;