[PATCH v2 4/7] binfmt: remove in-tree usage of MAP_DENYWRITE

[David Hildenbrand <david@redhat.com>]

At exec time when we mmap the new executable via MAP_DENYWRITE we have it
opened via do_open_execat() and already deny_write_access()'ed the file
successfully. Once exec completes, we allow_write_acces(); however,
we set mm->exe_file in begin_new_exec() via set_mm_exe_file() and
also deny_write_access() as long as mm->exe_file remains set. We'll
effectively deny write access to our executable via mm->exe_file
until mm->exe_file is changed -- when the process is removed, on new
exec, or via sys_prctl(PR_SET_MM_MAP/EXE_FILE).

Let's remove all usage of MAP_DENYWRITE, it's no longer necessary for
mm->exe_file.

In case of an elf interpreter, we'll now only deny write access to the file
during exec. This is somewhat okay, because the interpreter behaves
(and sometime is) a shared library; all shared libraries, especially the
ones loaded directly in user space like via dlopen() won't ever be mapped
via MAP_DENYWRITE, because we ignore that from user space completely;
these shared libraries can always be modified while mapped and executed.
Let's only special-case the main executable, denying write access while
being executed by a process. This can be considered a minor user space
visible change.

While this is a cleanup, it also fixes part of a problem reported with
VM_DENYWRITE on overlayfs, as VM_DENYWRITE is effectively unused with
this patch and will be removed next:
  "Overlayfs did not honor positive i_writecount on realfile for
   VM_DENYWRITE mappings." [1]

[1] https://lore.kernel.org/r/YNHXzBgzRrZu1MrD@miu.piliscsaba.redhat.com/

Reported-by: Chengguang Xu <cgxu519@mykernel.net>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 arch/x86/ia32/ia32_aout.c | 6 ++----
 fs/binfmt_aout.c          | 5 ++---
 fs/binfmt_elf.c           | 4 ++--
 fs/binfmt_elf_fdpic.c     | 2 +-
 4 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/arch/x86/ia32/ia32_aout.c b/arch/x86/ia32/ia32_aout.c
index 321d7b22ad2d..9bd15241fadb 100644
--- a/arch/x86/ia32/ia32_aout.c
+++ b/arch/x86/ia32/ia32_aout.c
@@ -202,8 +202,7 @@ static int load_aout_binary(struct linux_binprm *bprm)
 
 		error = vm_mmap(bprm->file, N_TXTADDR(ex), ex.a_text,
 				PROT_READ | PROT_EXEC,
-				MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE |
-				MAP_32BIT,
+				MAP_FIXED | MAP_PRIVATE | MAP_32BIT,
 				fd_offset);
 
 		if (error != N_TXTADDR(ex))
@@ -211,8 +210,7 @@ static int load_aout_binary(struct linux_binprm *bprm)
 
 		error = vm_mmap(bprm->file, N_DATADDR(ex), ex.a_data,
 				PROT_READ | PROT_WRITE | PROT_EXEC,
-				MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE |
-				MAP_32BIT,
+				MAP_FIXED | MAP_PRIVATE | MAP_32BIT,
 				fd_offset + ex.a_text);
 		if (error != N_DATADDR(ex))
 			return error;
diff --git a/fs/binfmt_aout.c b/fs/binfmt_aout.c
index d29de971d3f3..a47496d0f123 100644
--- a/fs/binfmt_aout.c
+++ b/fs/binfmt_aout.c
@@ -221,8 +221,7 @@ static int load_aout_binary(struct linux_binprm * bprm)
 		}
 
 		error = vm_mmap(bprm->file, N_TXTADDR(ex), ex.a_text,
-			PROT_READ | PROT_EXEC,
-			MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE,
+			PROT_READ | PROT_EXEC, MAP_FIXED | MAP_PRIVATE,
 			fd_offset);
 
 		if (error != N_TXTADDR(ex))
@@ -230,7 +229,7 @@ static int load_aout_binary(struct linux_binprm * bprm)
 
 		error = vm_mmap(bprm->file, N_DATADDR(ex), ex.a_data,
 				PROT_READ | PROT_WRITE | PROT_EXEC,
-				MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE,
+				MAP_FIXED | MAP_PRIVATE,
 				fd_offset + ex.a_text);
 		if (error != N_DATADDR(ex))
 			return error;
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 6d2c79533631..69d900a8473d 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -622,7 +622,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 	eppnt = interp_elf_phdata;
 	for (i = 0; i < interp_elf_ex->e_phnum; i++, eppnt++) {
 		if (eppnt->p_type == PT_LOAD) {
-			int elf_type = MAP_PRIVATE | MAP_DENYWRITE;
+			int elf_type = MAP_PRIVATE;
 			int elf_prot = make_prot(eppnt->p_flags, arch_state,
 						 true, true);
 			unsigned long vaddr = 0;
@@ -1070,7 +1070,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		elf_prot = make_prot(elf_ppnt->p_flags, &arch_state,
 				     !!interpreter, false);
 
-		elf_flags = MAP_PRIVATE | MAP_DENYWRITE;
+		elf_flags = MAP_PRIVATE;
 
 		vaddr = elf_ppnt->p_vaddr;
 		/*
diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index cf4028487dcc..6d8fd6030cbb 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -1041,7 +1041,7 @@ static int elf_fdpic_map_file_by_direct_mmap(struct elf_fdpic_params *params,
 		if (phdr->p_flags & PF_W) prot |= PROT_WRITE;
 		if (phdr->p_flags & PF_X) prot |= PROT_EXEC;
 
-		flags = MAP_PRIVATE | MAP_DENYWRITE;
+		flags = MAP_PRIVATE;
 		maddr = 0;
 
 		switch (params->flags & ELF_FDPIC_FLAG_ARRANGEMENT) {
-- 
2.31.1