Commit d2cd9ede authored Sep 06, 2017 by Rik van Riel Committed by Linus Torvalds Sep 06, 2017

mm,fork: introduce MADV_WIPEONFORK

Introduce MADV_WIPEONFORK semantics, which result in a VMA being empty
in the child process after fork.  This differs from MADV_DONTFORK in one
important way.

If a child process accesses memory that was MADV_WIPEONFORK, it will get
zeroes.  The address ranges are still valid, they are just empty.

If a child process accesses memory that was MADV_DONTFORK, it will get a
segmentation fault, since those address ranges are no longer valid in
the child after fork.

Since MADV_DONTFORK also seems to be used to allow very large programs
to fork in systems with strict memory overcommit restrictions, changing
the semantics of MADV_DONTFORK might break existing programs.

MADV_WIPEONFORK only works on private, anonymous VMAs.

The use case is libraries that store or cache information, and want to
know that they need to regenerate it in the child process after fork.

Examples of this would be:
 - systemd/pulseaudio API checks (fail after fork) (replacing a getpid
   check, which is too slow without a PID cache)
 - PKCS#11 API reinitialization check (mandated by specification)
 - glibc's upcoming PRNG (reseed after fork)
 - OpenSSL PRNG (reseed after fork)

The security benefits of a forking server having a re-inialized PRNG in
every child process are pretty obvious.  However, due to libraries
having all kinds of internal state, and programs getting compiled with
many different versions of each library, it is unreasonable to expect
calling programs to re-initialize everything manually after fork.

A further complication is the proliferation of clone flags, programs
bypassing glibc's functions to call clone directly, and programs calling
unshare, causing the glibc pthread_atfork hook to not get called.

It would be better to have the kernel take care of this automatically.

The patch also adds MADV_KEEPONFORK, to undo the effects of a prior
MADV_WIPEONFORK.

This is similar to the OpenBSD minherit syscall with MAP_INHERIT_ZERO:

    https://man.openbsd.org/minherit.2

[akpm@linux-foundation.org: numerically order arch/parisc/include/uapi/asm/mman.h #defines]
Link: http://lkml.kernel.org/r/20170811212829.29186-3-riel@redhat.com


Signed-off-by: Rik van Riel <riel@redhat.com>
Reported-by: Florian Weimer <fweimer@redhat.com>
Reported-by: Colm MacCártaigh <colm@allcosts.net>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Helge Deller <deller@gmx.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Cc: <linux-api@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent df3735c5

arch/alpha/include/uapi/asm/mman.h

+3 −0

Original line number	Diff line number	Diff line
		@@ -64,6 +64,9 @@
		overrides the coredump filter bits */
		#define MADV_DODUMP 17 /* Clear the MADV_NODUMP flag */

		#define MADV_WIPEONFORK 18 /* Zero memory on fork, child only */
		#define MADV_KEEPONFORK 19 /* Undo MADV_WIPEONFORK */

		/* compatibility flags */
		#define MAP_FILE 0

arch/mips/include/uapi/asm/mman.h

+3 −0

Original line number	Diff line number	Diff line
		@@ -91,6 +91,9 @@
		overrides the coredump filter bits */
		#define MADV_DODUMP 17 /* Clear the MADV_NODUMP flag */

		#define MADV_WIPEONFORK 18 /* Zero memory on fork, child only */
		#define MADV_KEEPONFORK 19 /* Undo MADV_WIPEONFORK */

		/* compatibility flags */
		#define MAP_FILE 0

arch/parisc/include/uapi/asm/mman.h

+3 −0

Original line number	Diff line number	Diff line
		@@ -57,6 +57,9 @@
		overrides the coredump filter bits */
		#define MADV_DODUMP 70 /* Clear the MADV_NODUMP flag */

		#define MADV_WIPEONFORK 71 /* Zero memory on fork, child only */
		#define MADV_KEEPONFORK 72 /* Undo MADV_WIPEONFORK */

		#define MADV_HWPOISON 100 /* poison a page for testing */
		#define MADV_SOFT_OFFLINE 101 /* soft offline page for testing */

arch/xtensa/include/uapi/asm/mman.h

+3 −0

Original line number	Diff line number	Diff line
		@@ -103,6 +103,9 @@
		overrides the coredump filter bits */
		#define MADV_DODUMP 17 /* Clear the MADV_NODUMP flag */

		#define MADV_WIPEONFORK 18 /* Zero memory on fork, child only */
		#define MADV_KEEPONFORK 19 /* Undo MADV_WIPEONFORK */

		/* compatibility flags */
		#define MAP_FILE 0

fs/proc/task_mmu.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -663,6 +663,7 @@ static void show_smap_vma_flags(struct seq_file m, struct vm_area_struct vma)
		[ilog2(VM_NORESERVE)] = "nr",
		[ilog2(VM_HUGETLB)] = "ht",
		[ilog2(VM_ARCH_1)] = "ar",
		[ilog2(VM_WIPEONFORK)] = "wf",
		[ilog2(VM_DONTDUMP)] = "dd",
		#ifdef CONFIG_MEM_SOFT_DIRTY
		[ilog2(VM_SOFTDIRTY)] = "sd",

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