Re: [PATCH] mm/nommu.c: Fix improperly call of security API in mmap

[David Howells]

<graff.yang@xxxxxxxxx> wrote:

> The original code calling security_file_mmap() use user's hint address
> as it's 5th argument(addr). This is improper, as the hint address may be
> NULL.
> In this case, the security_file_mmap() may incorrectly return -EPERM.
> 
> This patch moved the calling of security_file_mmap() out of
> validate_mmap_request() to do_mmap_pgoff(), and call this
> security API with the address that attempting to mmap.

I think this is the wrong approach.  Firstly, the hint is cleared in NOMMU
mode, and secondly, I think that the check against the minimum LSM address is
pointless in NOMMU mode too.

So I think the attached patch is a better approach.

David
---
From: David Howells <dhowells@xxxxxxxxxx>

NOMMU: Ignore the address parameter in the file_mmap() security check

Ignore the address parameter in the various file_mmap() security checks when
CONFIG_MMU=n as the address hint is ignored under those circumstances, and in
any case the minimum mapping address check is pointless in NOMMU mode.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 include/linux/security.h |    1 +
 mm/nommu.c               |    2 +-
 security/commoncap.c     |    2 ++
 security/selinux/hooks.c |    2 ++
 4 files changed, 6 insertions(+), 1 deletions(-)


diff --git a/include/linux/security.h b/include/linux/security.h
index 239e40d..0583f16 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -593,6 +593,7 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
  *	@reqprot contains the protection requested by the application.
  *	@prot contains the protection that will be applied by the kernel.
  *	@flags contains the operational flags.
+ *	@addr contains the mapping address, and should be ignored in NOMMU mode.
  *	Return 0 if permission is granted.
  * @file_mprotect:
  *	Check permissions before changing memory access permissions.
diff --git a/mm/nommu.c b/mm/nommu.c
index 3c3b4b2..cfea46c 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -974,7 +974,7 @@ static int validate_mmap_request(struct file *file,
 	}
 
 	/* allow the security API to have its say */
-	ret = security_file_mmap(file, reqprot, prot, flags, addr, 0);
+	ret = security_file_mmap(file, reqprot, prot, flags, 0, 0);
 	if (ret < 0)
 		return ret;
 
diff --git a/security/commoncap.c b/security/commoncap.c
index fe30751..ac1f745 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -1005,6 +1005,7 @@ int cap_file_mmap(struct file *file, unsigned long reqprot,
 {
 	int ret = 0;
 
+#ifdef CONFIG_MMU
 	if (addr < dac_mmap_min_addr) {
 		ret = cap_capable(current, current_cred(), CAP_SYS_RAWIO,
 				  SECURITY_CAP_AUDIT);
@@ -1012,5 +1013,6 @@ int cap_file_mmap(struct file *file, unsigned long reqprot,
 		if (ret == 0)
 			current->flags |= PF_SUPERPRIV;
 	}
+#endif
 	return ret;
 }
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index bb230d5..93d61f8 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3046,6 +3046,7 @@ static int selinux_file_mmap(struct file *file, unsigned long reqprot,
 			     unsigned long addr, unsigned long addr_only)
 {
 	int rc = 0;
+#ifdef CONFIG_MMU
 	u32 sid = current_sid();
 
 	/*
@@ -3060,6 +3061,7 @@ static int selinux_file_mmap(struct file *file, unsigned long reqprot,
 		if (rc)
 			return rc;
 	}
+#endif
 
 	/* do DAC check on address space usage */
 	rc = cap_file_mmap(file, reqprot, prot, flags, addr, addr_only);
--
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