linux-kernel - [PATCH] staging: android: ashmem: Fix mmap size validation

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Message-Id: <20180619222444.134928-1-astrachan@google.com>
Date:   Tue, 19 Jun 2018 15:24:44 -0700
From:   Alistair Strachan <astrachan@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Alistair Strachan <astrachan@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Arve Hjønnevåg" <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>, devel@...verdev.osuosl.org,
        kernel-team@...roid.com
Subject: [PATCH] staging: android: ashmem: Fix mmap size validation

The ashmem driver did not check that the size/offset of the vma passed
to its .mmap() function was not larger than the ashmem object being
mapped. This could cause mmap() to succeed, even though accessing parts
of the mapping would later fail with a segmentation fault.

Ensure an error is returned by the ashmem_mmap() function if the vma
size is larger than the ashmem object size. This enables safer handling
of the problem in userspace.

Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Arve Hjønnevåg <arve@...roid.com>
Cc: Todd Kjos <tkjos@...roid.com>
Cc: Martijn Coenen <maco@...roid.com>
Cc: devel@...verdev.osuosl.org
Cc: kernel-team@...roid.com
Signed-off-by: Alistair Strachan <astrachan@...gle.com>
---
 drivers/staging/android/ashmem.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index a1a0025b59e0..1eeedb529a10 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -366,6 +366,12 @@ static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
 		goto out;
 	}
 
+	/* requested mapping size larger than object size */
+	if (unlikely(vma->vm_end - vma->vm_start > PAGE_ALIGN(asma->size))) {
+		ret = -EINVAL;
+		goto out;
+	}
+
 	/* requested protection bits must match our allowed protection mask */
 	if (unlikely((vma->vm_flags & ~calc_vm_prot_bits(asma->prot_mask, 0)) &
 		     calc_vm_prot_bits(PROT_MASK, 0))) {