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Date: Fri, 15 Dec 2017 16:49:28 -0800
From: Andrei Vagin <avagin@...tuozzo.com>
To: Michal Hocko <mhocko@...nel.org>
Cc: linux-api@...r.kernel.org, Khalid Aziz <khalid.aziz@...cle.com>,
Michael Ellerman <mpe@...erman.id.au>,
Andrew Morton <akpm@...ux-foundation.org>,
Russell King - ARM Linux <linux@...linux.org.uk>,
Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
Florian Weimer <fweimer@...hat.com>,
John Hubbard <jhubbard@...dia.com>,
Matthew Wilcox <willy@...radead.org>,
Michal Hocko <mhocko@...e.com>,
Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
Joel Stanley <joel@....id.au>,
Kees Cook <keescook@...omium.org>
Subject: Re: [2/2] fs, elf: drop MAP_FIXED usage from elf_map
Hi Michal,
We run CRIU tests for linux-next and the 4.15.0-rc3-next-20171215 kernel
doesn't boot:
[ 3.492549] Freeing unused kernel memory: 1640K
[ 3.494547] Write protecting the kernel read-only data: 18432k
[ 3.498781] Freeing unused kernel memory: 2016K
[ 3.503330] Freeing unused kernel memory: 512K
[ 3.505232] rodata_test: all tests were successful
[ 3.515355] 1 (init): Uhuuh, elf segement at 00000000928fda3e requested but the memory is mapped already
[ 3.519533] Starting init: /sbin/init exists but couldn't execute it (error -95)
[ 3.528993] Starting init: /bin/sh exists but couldn't execute it (error -14)
[ 3.532127] Kernel panic - not syncing: No working init found. Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst for guidance.
[ 3.538328] CPU: 0 PID: 1 Comm: init Not tainted 4.15.0-rc3-next-20171215-00001-g6d6aea478fce #11
[ 3.542201] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1.fc26 04/01/2014
[ 3.546081] Call Trace:
[ 3.547221] dump_stack+0x5c/0x79
[ 3.548768] ? rest_init+0x30/0xb0
[ 3.550320] panic+0xe4/0x232
[ 3.551669] ? rest_init+0xb0/0xb0
[ 3.553110] kernel_init+0xeb/0x100
[ 3.554701] ret_from_fork+0x1f/0x30
[ 3.558964] Kernel Offset: 0x2000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[ 3.564160] ---[ end Kernel panic - not syncing: No working init found. Try passing init= option to kernel. See Linux Documentation/admin-guide/init.rst for guidance.
If I revert this patch, it boots normally.
Thanks,
Andrei
On Wed, Dec 13, 2017 at 10:25:50AM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
>
> Both load_elf_interp and load_elf_binary rely on elf_map to map segments
> on a controlled address and they use MAP_FIXED to enforce that. This is
> however dangerous thing prone to silent data corruption which can be
> even exploitable. Let's take CVE-2017-1000253 as an example. At the time
> (before eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE"))
> ELF_ET_DYN_BASE was at TASK_SIZE / 3 * 2 which is not that far away from
> the stack top on 32b (legacy) memory layout (only 1GB away). Therefore
> we could end up mapping over the existing stack with some luck.
>
> The issue has been fixed since then (a87938b2e246 ("fs/binfmt_elf.c:
> fix bug in loading of PIE binaries")), ELF_ET_DYN_BASE moved moved much
> further from the stack (eab09532d400 and later by c715b72c1ba4 ("mm:
> revert x86_64 and arm64 ELF_ET_DYN_BASE base changes")) and excessive
> stack consumption early during execve fully stopped by da029c11e6b1
> ("exec: Limit arg stack to at most 75% of _STK_LIM"). So we should be
> safe and any attack should be impractical. On the other hand this is
> just too subtle assumption so it can break quite easily and hard to
> spot.
>
> I believe that the MAP_FIXED usage in load_elf_binary (et. al) is still
> fundamentally dangerous. Moreover it shouldn't be even needed. We are
> at the early process stage and so there shouldn't be unrelated mappings
> (except for stack and loader) existing so mmap for a given address
> should succeed even without MAP_FIXED. Something is terribly wrong if
> this is not the case and we should rather fail than silently corrupt the
> underlying mapping.
>
> Address this issue by changing MAP_FIXED to the newly added
> MAP_FIXED_SAFE. This will mean that mmap will fail if there is an
> existing mapping clashing with the requested one without clobbering it.
>
> Cc: Abdul Haleem <abdhalee@...ux.vnet.ibm.com>
> Cc: Joel Stanley <joel@....id.au>
> Acked-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: Khalid Aziz <khalid.aziz@...cle.com>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
> arch/metag/kernel/process.c | 6 +++++-
> fs/binfmt_elf.c | 12 ++++++++----
> 2 files changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/arch/metag/kernel/process.c b/arch/metag/kernel/process.c
> index 0909834c83a7..867c8d0a5fb4 100644
> --- a/arch/metag/kernel/process.c
> +++ b/arch/metag/kernel/process.c
> @@ -399,7 +399,7 @@ unsigned long __metag_elf_map(struct file *filep, unsigned long addr,
> tcm_tag = tcm_lookup_tag(addr);
>
> if (tcm_tag != TCM_INVALID_TAG)
> - type &= ~MAP_FIXED;
> + type &= ~(MAP_FIXED | MAP_FIXED_SAFE);
>
> /*
> * total_size is the size of the ELF (interpreter) image.
> @@ -417,6 +417,10 @@ unsigned long __metag_elf_map(struct file *filep, unsigned long addr,
> } else
> map_addr = vm_mmap(filep, addr, size, prot, type, off);
>
> + if ((type & MAP_FIXED_SAFE) && BAD_ADDR(map_addr))
> + pr_info("%d (%s): Uhuuh, elf segement at %p requested but the memory is mapped already\n",
> + task_pid_nr(current), tsk->comm, (void*)addr);
> +
> if (!BAD_ADDR(map_addr) && tcm_tag != TCM_INVALID_TAG) {
> struct tcm_allocation *tcm;
> unsigned long tcm_addr;
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 73b01e474fdc..5916d45f64a7 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -372,6 +372,10 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
> } else
> map_addr = vm_mmap(filep, addr, size, prot, type, off);
>
> + if ((type & MAP_FIXED_SAFE) && BAD_ADDR(map_addr))
> + pr_info("%d (%s): Uhuuh, elf segement at %p requested but the memory is mapped already\n",
> + task_pid_nr(current), current->comm, (void*)addr);
> +
> return(map_addr);
> }
>
> @@ -569,7 +573,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
> elf_prot |= PROT_EXEC;
> vaddr = eppnt->p_vaddr;
> if (interp_elf_ex->e_type == ET_EXEC || load_addr_set)
> - elf_type |= MAP_FIXED;
> + elf_type |= MAP_FIXED_SAFE;
> else if (no_base && interp_elf_ex->e_type == ET_DYN)
> load_addr = -vaddr;
>
> @@ -930,7 +934,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
> * the ET_DYN load_addr calculations, proceed normally.
> */
> if (loc->elf_ex.e_type == ET_EXEC || load_addr_set) {
> - elf_flags |= MAP_FIXED;
> + elf_flags |= MAP_FIXED_SAFE;
> } else if (loc->elf_ex.e_type == ET_DYN) {
> /*
> * This logic is run once for the first LOAD Program
> @@ -966,7 +970,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
> load_bias = ELF_ET_DYN_BASE;
> if (current->flags & PF_RANDOMIZE)
> load_bias += arch_mmap_rnd();
> - elf_flags |= MAP_FIXED;
> + elf_flags |= MAP_FIXED_SAFE;
> } else
> load_bias = 0;
>
> @@ -1223,7 +1227,7 @@ static int load_elf_library(struct file *file)
> (eppnt->p_filesz +
> ELF_PAGEOFFSET(eppnt->p_vaddr)),
> PROT_READ | PROT_WRITE | PROT_EXEC,
> - MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE,
> + MAP_FIXED_SAFE | MAP_PRIVATE | MAP_DENYWRITE,
> (eppnt->p_offset -
> ELF_PAGEOFFSET(eppnt->p_vaddr)));
> if (error != ELF_PAGESTART(eppnt->p_vaddr))
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