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Message-ID: <87f80506-eeb3-4848-adc9-8a030b5f4136@arm.com>
Date: Fri, 2 May 2025 11:01:07 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Kees Cook <kees@...nel.org>
Cc: Al Viro <viro@...iv.linux.org.uk>, Christian Brauner
<brauner@...nel.org>, Jan Kara <jack@...e.cz>,
Ali Saidi <alisaidi@...zon.com>, Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] binfmt_elf: Move brk for static PIE even if ASLR
disabled
On 02/05/2025 01:18, Kees Cook wrote:
> In commit bbdc6076d2e5 ("binfmt_elf: move brk out of mmap when doing
> direct loader exec"), the brk was moved out of the mmap region when
> loading static PIE binaries (ET_DYN without INTERP). The common case
> for these binaries was testing new ELF loaders, so the brk needed to
> be away from mmap to avoid colliding with stack, future mmaps (of the
> loader-loaded binary), etc. But this was only done when ASLR was enabled,
> in an attempt to minimize changes to memory layouts.
>
> After adding support to respect alignment requirements for static PIE
> binaries in commit 3545deff0ec7 ("binfmt_elf: Honor PT_LOAD alignment
> for static PIE"), it became possible to have a large gap after the
> final PT_LOAD segment and the top of the mmap region. This means that
> future mmap allocations might go after the last PT_LOAD segment (where
> brk might be if ASLR was disabled) instead of before them (where they
> traditionally ended up).
>
> On arm64, running with ASLR disabled, Ubuntu 22.04's "ldconfig" binary,
> a static PIE, has alignment requirements that leaves a gap large enough
> after the last PT_LOAD segment to fit the vdso and vvar, but still leave
> enough space for the brk (which immediately follows the last PT_LOAD
> segment) to be allocated by the binary.
>
> fffff7f20000-fffff7fde000 r-xp 00000000 fe:02 8110426 /sbin/ldconfig.real
> fffff7fee000-fffff7ff5000 rw-p 000be000 fe:02 8110426 /sbin/ldconfig.real
> fffff7ff5000-fffff7ffa000 rw-p 00000000 00:00 0
> ***[brk will go here at fffff7ffa000]***
> fffff7ffc000-fffff7ffe000 r--p 00000000 00:00 0 [vvar]
> fffff7ffe000-fffff8000000 r-xp 00000000 00:00 0 [vdso]
> fffffffdf000-1000000000000 rw-p 00000000 00:00 0 [stack]
>
> After commit 0b3bc3354eb9 ("arm64: vdso: Switch to generic storage
> implementation"), the arm64 vvar grew slightly, and suddenly the brk
> collided with the allocation.
>
> fffff7f20000-fffff7fde000 r-xp 00000000 fe:02 8110426 /sbin/ldconfig.real
> fffff7fee000-fffff7ff5000 rw-p 000be000 fe:02 8110426 /sbin/ldconfig.real
> fffff7ff5000-fffff7ffa000 rw-p 00000000 00:00 0
> ***[oops, no room any more, vvar is at fffff7ffa000!]***
> fffff7ffa000-fffff7ffe000 r--p 00000000 00:00 0 [vvar]
> fffff7ffe000-fffff8000000 r-xp 00000000 00:00 0 [vdso]
> fffffffdf000-1000000000000 rw-p 00000000 00:00 0 [stack]
>
> The solution is to unconditionally move the brk out of the mmap region
> for static PIE binaries. Whether ASLR is enabled or not does not change if
> there may be future mmap allocation collisions with a growing brk region.
>
> Update memory layout comments (with kernel-doc headings), consolidate
> the setting of mm->brk to later (it isn't needed early), move static PIE
> brk out of mmap unconditionally, and make sure brk(2) knows to base brk
> position off of mm->start_brk not mm->end_data no matter what the cause of
> moving it is (via current->brk_randomized).
>
> For the CONFIG_COMPAT_BRK case, though, leave the logic unchanged, as we
> can never safely move the brk. These systems, however, are not using
> specially aligned static PIE binaries.
>
> Reported-by: Ryan Roberts <ryan.roberts@....com>
> Closes: https://lore.kernel.org/lkml/f93db308-4a0e-4806-9faf-98f890f5a5e6@arm.com/
> Fixes: bbdc6076d2e5 ("binfmt_elf: move brk out of mmap when doing direct loader exec")
> Link: https://lore.kernel.org/r/20250425224502.work.520-kees@kernel.org
> Signed-off-by: Kees Cook <kees@...nel.org>
It's a shame we can't figure out a universal solution that would work for
CONFIG_COMPAT_BRK too, but I can't think of anything. So as you say, let's worry
about that in the unlikely event that an issue is reported.
Reviewed-by: Ryan Roberts <ryan.roberts@....com>
Tested-by: Ryan Roberts <ryan.roberts@....com>
Thanks for sorting this out! Would be great to get it into v6.15.
> ---
> v2: exclude CONFIG_COMPAT_BRK (ryan)
> v1: https://lore.kernel.org/all/20250425224502.work.520-kees@kernel.org/
> ---
> fs/binfmt_elf.c | 71 ++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 47 insertions(+), 24 deletions(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 584fa89bc877..4c1ea6b52a53 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -830,6 +830,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
> struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
> struct elf_phdr *elf_property_phdata = NULL;
> unsigned long elf_brk;
> + bool brk_moved = false;
> int retval, i;
> unsigned long elf_entry;
> unsigned long e_entry;
> @@ -1097,15 +1098,19 @@ static int load_elf_binary(struct linux_binprm *bprm)
> /* Calculate any requested alignment. */
> alignment = maximum_alignment(elf_phdata, elf_ex->e_phnum);
>
> - /*
> - * There are effectively two types of ET_DYN
> - * binaries: programs (i.e. PIE: ET_DYN with PT_INTERP)
> - * and loaders (ET_DYN without PT_INTERP, since they
> - * _are_ the ELF interpreter). The loaders must
> - * be loaded away from programs since the program
> - * may otherwise collide with the loader (especially
> - * for ET_EXEC which does not have a randomized
> - * position). For example to handle invocations of
> + /**
> + * DOC: PIE handling
> + *
> + * There are effectively two types of ET_DYN ELF
> + * binaries: programs (i.e. PIE: ET_DYN with
> + * PT_INTERP) and loaders (i.e. static PIE: ET_DYN
> + * without PT_INTERP, usually the ELF interpreter
> + * itself). Loaders must be loaded away from programs
> + * since the program may otherwise collide with the
> + * loader (especially for ET_EXEC which does not have
> + * a randomized position).
> + *
> + * For example, to handle invocations of
> * "./ld.so someprog" to test out a new version of
> * the loader, the subsequent program that the
> * loader loads must avoid the loader itself, so
> @@ -1118,6 +1123,9 @@ static int load_elf_binary(struct linux_binprm *bprm)
> * ELF_ET_DYN_BASE and loaders are loaded into the
> * independently randomized mmap region (0 load_bias
> * without MAP_FIXED nor MAP_FIXED_NOREPLACE).
> + *
> + * See below for "brk" handling details, which is
> + * also affected by program vs loader and ASLR.
> */
> if (interpreter) {
> /* On ET_DYN with PT_INTERP, we do the ASLR. */
> @@ -1234,8 +1242,6 @@ static int load_elf_binary(struct linux_binprm *bprm)
> start_data += load_bias;
> end_data += load_bias;
>
> - current->mm->start_brk = current->mm->brk = ELF_PAGEALIGN(elf_brk);
> -
> if (interpreter) {
> elf_entry = load_elf_interp(interp_elf_ex,
> interpreter,
> @@ -1291,27 +1297,44 @@ static int load_elf_binary(struct linux_binprm *bprm)
> mm->end_data = end_data;
> mm->start_stack = bprm->p;
>
> - if ((current->flags & PF_RANDOMIZE) && (snapshot_randomize_va_space > 1)) {
> + /**
> + * DOC: "brk" handling
> + *
> + * For architectures with ELF randomization, when executing a
> + * loader directly (i.e. static PIE: ET_DYN without PT_INTERP),
> + * move the brk area out of the mmap region and into the unused
> + * ELF_ET_DYN_BASE region. Since "brk" grows up it may collide
> + * early with the stack growing down or other regions being put
> + * into the mmap region by the kernel (e.g. vdso).
> + *
> + * In the CONFIG_COMPAT_BRK case, though, everything is turned
> + * off because we're not allowed to move the brk at all.
> + */
> + if (!IS_ENABLED(CONFIG_COMPAT_BRK) &&
> + IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) &&
> + elf_ex->e_type == ET_DYN && !interpreter) {
> + elf_brk = ELF_ET_DYN_BASE;
> + /* This counts as moving the brk, so let brk(2) know. */
> + brk_moved = true;
> + }
> + mm->start_brk = mm->brk = ELF_PAGEALIGN(elf_brk);
> +
> + if ((current->flags & PF_RANDOMIZE) && snapshot_randomize_va_space > 1) {
> /*
> - * For architectures with ELF randomization, when executing
> - * a loader directly (i.e. no interpreter listed in ELF
> - * headers), move the brk area out of the mmap region
> - * (since it grows up, and may collide early with the stack
> - * growing down), and into the unused ELF_ET_DYN_BASE region.
> + * If we didn't move the brk to ELF_ET_DYN_BASE (above),
> + * leave a gap between .bss and brk.
> */
> - if (IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) &&
> - elf_ex->e_type == ET_DYN && !interpreter) {
> - mm->brk = mm->start_brk = ELF_ET_DYN_BASE;
> - } else {
> - /* Otherwise leave a gap between .bss and brk. */
> + if (!brk_moved)
> mm->brk = mm->start_brk = mm->brk + PAGE_SIZE;
> - }
>
> mm->brk = mm->start_brk = arch_randomize_brk(mm);
> + brk_moved = true;
> + }
> +
> #ifdef compat_brk_randomized
> + if (brk_moved)
> current->brk_randomized = 1;
> #endif
> - }
>
> if (current->personality & MMAP_PAGE_ZERO) {
> /* Why this, you ask??? Well SVr4 maps page 0 as read-only,
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