linux-kernel - [PATCH 26/36] fork: Define usercopy region in mm

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Message-Id: <1515531365-37423-27-git-send-email-keescook@chromium.org>
Date:   Tue,  9 Jan 2018 12:55:55 -0800
From:   Kees Cook <keescook@...omium.org>
To:     linux-kernel@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        David Windsor <dave@...lcore.net>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Christoph Lameter <cl@...ux.com>,
        "David S. Miller" <davem@...emloft.net>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        Jan Kara <jack@...e.cz>,
        Luis de Bethencourt <luisbg@...nel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Rik van Riel <riel@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        kernel-hardening@...ts.openwall.com
Subject: [PATCH 26/36] fork: Define usercopy region in mm_struct slab caches

From: David Windsor <dave@...lcore.net>

In support of usercopy hardening, this patch defines a region in the
mm_struct slab caches in which userspace copy operations are allowed.
Only the auxv field is copied to userspace.

cache object allocation:
    kernel/fork.c:
        #define allocate_mm()     (kmem_cache_alloc(mm_cachep, GFP_KERNEL))

        dup_mm():
            ...
            mm = allocate_mm();

        copy_mm(...):
            ...
            dup_mm();

        copy_process(...):
            ...
            copy_mm(...)

        _do_fork(...):
            ...
            copy_process(...)

example usage trace:

    fs/binfmt_elf.c:
        create_elf_tables(...):
            ...
            elf_info = (elf_addr_t *)current->mm->saved_auxv;
            ...
            copy_to_user(..., elf_info, ei_index * sizeof(elf_addr_t))

        load_elf_binary(...):
            ...
            create_elf_tables(...);

This region is known as the slab cache's usercopy region. Slab caches
can now check that each dynamically sized copy operation involving
cache-managed memory falls entirely within the slab's usercopy region.

This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY
whitelisting code in the last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code are
mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: David Windsor <dave@...lcore.net>
[kees: adjust commit log, split patch, provide usage trace]
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Kees Cook <keescook@...omium.org>
Acked-by: Rik van Riel <riel@...hat.com>
---
 kernel/fork.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 432eadf6b58c..82f2a0441d3b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2225,9 +2225,11 @@ void __init proc_caches_init(void)
 	 * maximum number of CPU's we can ever have.  The cpumask_allocation
 	 * is at the end of the structure, exactly for that reason.
 	 */
-	mm_cachep = kmem_cache_create("mm_struct",
+	mm_cachep = kmem_cache_create_usercopy("mm_struct",
 			sizeof(struct mm_struct), ARCH_MIN_MMSTRUCT_ALIGN,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT,
+			offsetof(struct mm_struct, saved_auxv),
+			sizeof_field(struct mm_struct, saved_auxv),
 			NULL);
 	vm_area_cachep = KMEM_CACHE(vm_area_struct, SLAB_PANIC|SLAB_ACCOUNT);
 	mmap_init();
-- 
2.7.4