[PATCH v2 1/7] binfmt: don't use MAP_DENYWRITE when loading shared libraries via uselib()

David Hildenbrand
2021-08-16T19:49:44Z

uselib() is the legacy systemcall for loading shared libraries.
Nowadays, applications use dlopen() to load shared libraries, completely
implemented in user space via mmap().

For example, glibc uses MAP_COPY to mmap shared libraries. While this
maps to MAP_PRIVATE | MAP_DENYWRITE on Linux, Linux ignores any
MAP_DENYWRITE specification from user space in mmap.

With this change, all remaining in-tree users of MAP_DENYWRITE use it
to map an executable. We will be able to open shared libraries loaded
via uselib() writable, just as we already can via dlopen() from user
space.

This is one step into the direction of removing MAP_DENYWRITE from the
kernel. This can be considered a minor user space visible change.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
arch/x86/ia32/ia32_aout.c | 2 +-
fs/binfmt_aout.c | 2 +-
fs/binfmt_elf.c | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/ia32/ia32_aout.c b/arch/x86/ia32/ia32_aout.c
index 5e5b9fc2747f..321d7b22ad2d 100644
--- a/arch/x86/ia32/ia32_aout.c
+++ b/arch/x86/ia32/ia32_aout.c
@@ -293,7 +293,7 @@ static int load_aout_library(struct file *file)
/* Now use mmap to map the library into memory. */
error = vm_mmap(file, start_addr, ex.a_text + ex.a_data,
PROT_READ | PROT_WRITE | PROT_EXEC,
- MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE | MAP_32BIT,
+ MAP_FIXED | MAP_PRIVATE | MAP_32BIT,
N_TXTOFF(ex));
retval = error;
if (error != start_addr)
diff --git a/fs/binfmt_aout.c b/fs/binfmt_aout.c
index 145917f734fe..d29de971d3f3 100644
--- a/fs/binfmt_aout.c
+++ b/fs/binfmt_aout.c
@@ -309,7 +309,7 @@ static int load_aout_library(struct file *file)
/* Now use mmap to map the library into memory. */
error = vm_mmap(file, start_addr, ex.a_text + ex.a_data,
PROT_READ | PROT_WRITE | PROT_EXEC,
- MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE,
+ MAP_FIXED | MAP_PRIVATE;
N_TXTOFF(ex));
retval = error;
if (error != start_addr)
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 439ed81e755a..6d2c79533631 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1384,7 +1384,7 @@ static int load_elf_library(struct file *file)
(eppnt->p_filesz +
ELF_PAGEOFFSET(eppnt->p_vaddr)),
PROT_READ | PROT_WRITE | PROT_EXEC,
- MAP_FIXED_NOREPLACE | MAP_PRIVATE | MAP_DENYWRITE,
+ MAP_FIXED_NOREPLACE | MAP_PRIVATE,
(eppnt->p_offset -
ELF_PAGEOFFSET(eppnt->p_vaddr)));
if (error != ELF_PAGESTART(eppnt->p_vaddr))
--
2.31.1