[PATCH] intel_th: Fix a missing-check bug

[Wenwen Wang <wang6495@umn.edu>]

In msc_data_sz(), the 'valid_dw' field of the msc block descriptor 'bdesc'
is firstly checked to see whether the descriptor has a valid data width. If
yes, i.e., 'bdesc->valid_dw' is not 0, the data size of this descriptor
will be returned. It is worth noting that the data size is calculated from
'bdesc->valid_dw'. The problem here is that 'bdesc' actually points to a
consistent DMA region, which is allocated through dma_alloc_coherent() in
msc_buffer_win_alloc(). Given that the device also has the permission to
access this DMA region, it is possible that a malicious device controlled
by an attacker can modify the 'valid_dw' field after the if statement but
before the return statement in msc_data_sz(). Through this way, the device
can bypass the check and supply unexpected data width.

This patch copies the 'valid_dw' field to a local variable and then
performs the check and the calculation on the local variable to avoid the
above issue.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
---
 drivers/hwtracing/intel_th/msu.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/hwtracing/intel_th/msu.h b/drivers/hwtracing/intel_th/msu.h
index 9cc8ace..b7d846e 100644
--- a/drivers/hwtracing/intel_th/msu.h
+++ b/drivers/hwtracing/intel_th/msu.h
@@ -79,10 +79,12 @@ struct msc_block_desc {
 
 static inline unsigned long msc_data_sz(struct msc_block_desc *bdesc)
 {
-	if (!bdesc->valid_dw)
+	u32 valid_dw = bdesc->valid_dw;
+
+	if (!valid_dw)
 		return 0;
 
-	return bdesc->valid_dw * 4 - MSC_BDESC;
+	return valid_dw * 4 - MSC_BDESC;
 }
 
 static inline bool msc_block_wrapped(struct msc_block_desc *bdesc)
-- 
2.7.4