6.12-stable review patch. If anyone has any objections, please let me know.
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From: Kees Cook kees@kernel.org
[ Upstream commit 11854fe263eb1b9a8efa33b0c087add7719ea9b4 ]
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@arm.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 Reviewed-by: Ryan Roberts ryan.roberts@arm.com Tested-by: Ryan Roberts ryan.roberts@arm.com Signed-off-by: Kees Cook kees@kernel.org Signed-off-by: Sasha Levin sashal@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 0a216a078c315..47335a0f4a618 100644 --- a/fs/binfmt_elf.c +++ b/fs/binfmt_elf.c @@ -825,6 +825,7 @@ 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; @@ -1092,15 +1093,19 @@ 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 @@ -1113,6 +1118,9 @@ 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. */ @@ -1229,8 +1237,6 @@ 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, @@ -1286,27 +1292,44 @@ 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,