[RFC PATCH v2 4/8] mseal: Check seal flag for mprotect(2)

[jeffxu@chromium.org]

From: Jeff Xu <jeffxu@google.com>

mprotect(2) changes protection of VMAs in the given address
range. Sealing will prevent unintended mprotect call.

What this patch does:
When a mprotect(2) is invoked, if one of its VMAs has MM_SEAL_MPROTECT
set from previous mseal(2) call, this mprotect(2) will fail, without
any VMA modified.

This patch is based on following:
1. do_mprotect_pkey() currently called in two places:
SYSCALL_DEFINE3(mprotect,...)
SYSCALL_DEFINE4(pkey_mprotect, ...)
and not in any other places, therefore omit changing the signature of
do_mprotect_pkey(), i.e. not passing checkSeals flag.

2. In do_mprotect_pkey(), calls can_modify_mm() before any
update is made on the VMAs.

Signed-off-by: Jeff Xu <jeffxu@google.com>
---
 mm/mprotect.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 130db91d3a8c..6321c4d0aa3f 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -32,6 +32,7 @@
 #include <linux/sched/sysctl.h>
 #include <linux/userfaultfd_k.h>
 #include <linux/memory-tiers.h>
+#include <uapi/linux/mman.h>
 #include <asm/cacheflush.h>
 #include <asm/mmu_context.h>
 #include <asm/tlbflush.h>
@@ -753,6 +754,20 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 		}
 	}
 
+       /*
+	* do_mprotect_pkey() currently called from two places:
+	* SYSCALL_DEFINE3(mprotect,...)
+	* SYSCALL_DEFINE4(pkey_mprotect, ...)
+	* and not from other places.
+	* Therefore, omit changing the signature of do_mprotect_pkey().
+	* Otherwise, we might need to add checkSeals and pass it
+	* from all callers of do_mprotect_pkey().
+	*/
+	if (!can_modify_mm(current->mm, start, end, MM_SEAL_MPROTECT)) {
+		error = -EACCES;
+		goto out;
+	}
+
 	prev = vma_prev(&vmi);
 	if (start > vma->vm_start)
 		prev = vma;
-- 
2.42.0.655.g421f12c284-goog