linux-kernel - [PATCH 1/2 v2] fs/binfmt_elf: Use PT_LOAD p_align values for suitable start address.

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Date:   Fri, 31 Jul 2020 14:37:43 -0400
From:   Chris Kennelly <ckennelly@...gle.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Song Liu <songliubraving@...com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        Hugh Dickens <hughd@...gle.com>,
        Andrew Morton <akpm@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Chris Kennelly <ckennelly@...gle.com>
Subject: [PATCH 1/2 v2] fs/binfmt_elf: Use PT_LOAD p_align values for suitable
 start address.

The current ELF loading mechancism provides page-aligned mappings.  This
can lead to the program being loaded in a way unsuitable for
file-backed, transparent huge pages when handling PIE executables.

For binaries built with increased alignment, this limits the number of
bits usable for ASLR, but provides some randomization over using fixed
load addresses/non-PIE binaries.

Tested: verified program with -Wl,-z,max-page-size=0x200000 loading
Signed-off-by: Chris Kennelly <ckennelly@...gle.com>
---
 fs/binfmt_elf.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 9fe3b51c116a6..24e80302b497d 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -421,6 +421,25 @@ static int elf_read(struct file *file, void *buf, size_t len, loff_t pos)
 	return 0;
 }
 
+static unsigned long maximum_alignment(struct elf_phdr *cmds, int nr)
+{
+	unsigned long alignment = 0;
+	int i;
+
+	for (i = 0; i < nr; i++) {
+		if (cmds[i].p_type == PT_LOAD) {
+			/* skip non-power of two alignments */
+			if (cmds[i].p_align & (cmds[i].p_align - 1))
+				continue;
+			if (cmds[i].p_align > alignment)
+				alignment = cmds[i].p_align;
+		}
+	}
+
+	/* ensure we align to at least one page */
+	return ELF_PAGEALIGN(alignment);
+}
+
 /**
  * load_elf_phdrs() - load ELF program headers
  * @elf_ex:   ELF header of the binary whose program headers should be loaded
@@ -1008,6 +1027,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		int elf_prot, elf_flags;
 		unsigned long k, vaddr;
 		unsigned long total_size = 0;
+		unsigned long alignment;
 
 		if (elf_ppnt->p_type != PT_LOAD)
 			continue;
@@ -1086,6 +1106,10 @@ 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();
+				alignment = maximum_alignment(
+					elf_phdata, elf_ex->e_phnum);
+				if (alignment)
+					load_bias &= ~(alignment - 1);
 				elf_flags |= MAP_FIXED;
 			} else
 				load_bias = 0;
-- 
2.28.0.163.g6104cc2f0b6-goog