linux-kernel - [PATCH] get_user_pages(..., write==1, ...) may return with readable pte.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 13 Oct 2006 15:33:42 -0500
From:	Robin Holt <holt@....com>
To:	Hugh Dickins <hugh@...itas.com>, Nick Piggin <npiggin@...e.de>,
	Linus Torvalds <torvalds@...l.org>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] get_user_pages(..., write==1, ...) may return with readable pte.

Handle the case in get_user_pages() when a call to __handle_mm_fault()
inserts a writable pte, and a process doing dup_mmap converts it
to readable before get_user_pages() does the subsequent request to
follow_page().

Signed-off-by: Robin Holt <holt@....com>

---

Hugh, Nick, and Linus,

I think I have tripped over another flavor of a get_user_pages bug
we addressed back in 2005.  I do not have a test case to prove it is
the issue I am trying to address, but I have done as thorough a code
walk-through as I can.

Assume a pte is currently empty.  A first pthread is in the kernel on
a call path which is leading to get_user_pages.  A second pthread is
in the process of doing a fork.  The process doing get_user_pages()
gets into __handle_mm_fault() and grabs ptl just before the process
doing a fork attempts to grab the ptl to convert the pages to COW.
__handle_mm_fault() will insert the writable pte and unlock ptl then
return with VM_FAULT_WRITE set.  The process doing a fork then gets
the lock and starts converting the pte to RO/COW.  The get_user_pages()
process then clears FOLL_WRITE from foll_flags and calls follow_page()
without write, adds to the map count for the page, but does not have a
writable mapping.

Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c	2006-10-06 12:06:25.000000000 -0500
+++ linux-2.6/mm/memory.c	2006-10-13 15:06:38.286230638 -0500
@@ -1063,15 +1063,7 @@ int get_user_pages(struct task_struct *t
 				int ret;
 				ret = __handle_mm_fault(mm, vma, start,
 						foll_flags & FOLL_WRITE);
-				/*
-				 * The VM_FAULT_WRITE bit tells us that do_wp_page has
-				 * broken COW when necessary, even if maybe_mkwrite
-				 * decided not to set pte_write. We can thus safely do
-				 * subsequent page lookups as if they were reads.
-				 */
-				if (ret & VM_FAULT_WRITE)
-					foll_flags &= ~FOLL_WRITE;
-				
+
 				switch (ret & ~VM_FAULT_WRITE) {
 				case VM_FAULT_MINOR:
 					tsk->min_flt++;
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/