[RFC PATCH 4/5] exec, elf: require opt-in for accepting preserved mem

Anthony Yznaga
2020-07-27T17:03:26Z

Don't copy preserved VMAs to the binary being exec'd unless the binary has
a "preserved-mem-ok" ELF note.

Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
---
fs/binfmt_elf.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++
fs/exec.c | 17 +++++-----
include/linux/binfmts.h | 7 ++++-
3 files changed, 100 insertions(+), 8 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 6445a6dbdb1d..46248b7b0a75 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -683,6 +683,81 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
return error;
}

+#define NOTES_SZ SZ_1K
+#define PRESERVED_MEM_OK_STRING "preserved-mem-ok"
+#define SZ_PRESERVED_MEM_OK_STRING sizeof(PRESERVED_MEM_OK_STRING)
+
+static int parse_elf_note(struct linux_binprm *bprm, const char *data, size_t *off, size_t datasz)
+{
+ const struct elf_note *nhdr;
+ const char *name;
+ size_t o;
+
+ o = *off;
+ datasz -= o;
+
+ if (datasz < sizeof(*nhdr))
+ return -ENOEXEC;
+
+ nhdr = (const struct elf_note *)(data + o);
+ o += sizeof(*nhdr);
+ datasz -= sizeof(*nhdr);
+
+ /*
+ * Currently only the preserved-mem-ok elf note is of interest.
+ */
+ if (nhdr->n_type != 0x07c1feed)
+ goto next;
+
+ if (nhdr->n_namesz > SZ_PRESERVED_MEM_OK_STRING)
+ return -ENOEXEC;
+
+ name = data + o;
+ if (datasz < SZ_PRESERVED_MEM_OK_STRING ||
+ strncmp(name, PRESERVED_MEM_OK_STRING, SZ_PRESERVED_MEM_OK_STRING))
+ return -ENOEXEC;
+
+ bprm->accepts_preserved_mem = 1;
+
+next:
+ o += roundup(nhdr->n_namesz, 4) + roundup(nhdr->n_descsz, 4);
+ *off = o;
+
+ return 0;
+}
+
+static int parse_elf_notes(struct linux_binprm *bprm, struct elf_phdr *phdr)
+{
+ char *notes;
+ size_t notes_sz;
+ size_t off = 0;
+ int ret;
+
+ if (!phdr)
+ return 0;
+
+ notes_sz = phdr->p_filesz;
+ if ((notes_sz > NOTES_SZ) || (notes_sz < sizeof(struct elf_note)))
+ return -ENOEXEC;
+
+ notes = kvmalloc(notes_sz, GFP_KERNEL);
+ if (!notes)
+ return -ENOMEM;
+
+ ret = elf_read(bprm->file, notes, notes_sz, phdr->p_offset);
+ if (ret < 0)
+ goto out;
+
+ while (off < notes_sz) {
+ ret = parse_elf_note(bprm, notes, &off, notes_sz);
+ if (ret)
+ break;
+ }
+out:
+ kvfree(notes);
+ return ret;
+}
+
/*
* These are the functions used to load ELF style executables and shared
* libraries. There is no binary dependent code anywhere else.
@@ -801,6 +876,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
unsigned long error;
struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
struct elf_phdr *elf_property_phdata = NULL;
+ struct elf_phdr *elf_notes_phdata = NULL;
unsigned long elf_bss, elf_brk;
int bss_prot = 0;
int retval, i;
@@ -909,6 +985,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
executable_stack = EXSTACK_DISABLE_X;
break;

+ case PT_NOTE:
+ elf_notes_phdata = elf_ppnt;
+ break;
+
case PT_LOPROC ... PT_HIPROC:
retval = arch_elf_pt_proc(elf_ex, elf_ppnt,
bprm->file, false,
@@ -970,6 +1050,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
if (retval)
goto out_free_dentry;

+ retval = parse_elf_notes(bprm, elf_notes_phdata);
+ if (retval)
+ goto out_free_dentry;
+
/* Flush all traces of the currently running executable */
retval = begin_new_exec(bprm);
if (retval)
diff --git a/fs/exec.c b/fs/exec.c
index 1de09c4eef00..b2b046fec1f8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1088,10 +1088,11 @@ static int vma_dup_some(struct mm_struct *old_mm, struct mm_struct *new_mm)
* On success, this function returns with the mutex
* exec_update_mutex locked.
*/
-static int exec_mmap(struct mm_struct *mm)
+static int exec_mmap(struct linux_binprm *bprm)
{
struct task_struct *tsk;
struct mm_struct *old_mm, *active_mm;
+ struct mm_struct *mm = bprm->mm;
int ret;

/* Notify parent that we're no longer interested in the old VM */
@@ -1118,11 +1119,13 @@ static int exec_mmap(struct mm_struct *mm)
mutex_unlock(&tsk->signal->exec_update_mutex);
return -EINTR;
}
- ret = vma_dup_some(old_mm, mm);
- if (ret) {
- mmap_read_unlock(old_mm);
- mutex_unlock(&tsk->signal->exec_update_mutex);
- return ret;
+ if (bprm->accepts_preserved_mem) {
+ ret = vma_dup_some(old_mm, mm);
+ if (ret) {
+ mmap_read_unlock(old_mm);
+ mutex_unlock(&tsk->signal->exec_update_mutex);
+ return ret;
+ }
}
}

@@ -1386,7 +1389,7 @@ int begin_new_exec(struct linux_binprm * bprm)
* Release all of the old mmap stuff
*/
acct_arg_size(bprm, 0);
- retval = exec_mmap(bprm->mm);
+ retval = exec_mmap(bprm);
if (retval)
goto out;

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 4a20b7517dd0..6a66589454c8 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -41,7 +41,12 @@ struct linux_binprm {
* Set when errors can no longer be returned to the
* original userspace.
*/
- point_of_no_return:1;
+ point_of_no_return:1,
+ /*
+ * Set if the binary being exec'd will accept memory marked
+ * for preservation by the outgoing process.
+ */
+ accepts_preserved_mem:1;
#ifdef __alpha__
unsigned int taso:1;
#endif
--
1.8.3.1