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

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Message-Id: <20200724172016.608742-2-ckennelly@google.com>
Date:   Fri, 24 Jul 2020 13:20:16 -0400
From:   Chris Kennelly <ckennelly@...gle.com>
To:     Alexey Dobriyan <adobriyan@...il.com>,
        Song Liu <songliubraving@...com>,
        David Rientjes <rientjes@...gle.com>,
        Ian Rogers <irogers@...gle.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Chris Kennelly <ckennelly@...gle.com>
Subject: [PATCH 1/1] 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.

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 f4713ea76e827..83fadf66d25ef 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -418,6 +418,25 @@ 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
@@ -883,6 +902,7 @@ 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;
@@ -960,6 +980,10 @@ 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.rc0.105.gf9edc3c819-goog