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Date:   Thu, 22 Oct 2020 17:39:16 -0700
Message-Id: <20201023003916.82906-1-junaids@google.com>
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Subject: [PATCH] x86/mm/KASLR: Account for minimum padding when calculating entropy
From:   Junaid Shahid <junaids@google.com>
To:     dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org
Cc:     linux-kernel@vger.kernel.org
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Precedence: bulk

Subtract the minimum padding between regions from the initial
remain_entropy. Without this, the last region could potentially
overflow past vaddr_end if we happen to get a specific sequence
of random numbers (although extremely unlikely in practice).
The bug can be demonstrated by replacing the prandom_bytes_state
call with "rand = entropy;"

Signed-off-by: Junaid Shahid <junaids@google.com>
---
 arch/x86/mm/kaslr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 6e6b39710e5f..fe3eec30f736 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -109,7 +109,8 @@ void __init kernel_randomize_memory(void)
 	kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
 
 	/* Calculate entropy available between regions */
-	remain_entropy = vaddr_end - vaddr_start;
+	remain_entropy = vaddr_end - vaddr_start -
+			 (ARRAY_SIZE(kaslr_regions) - 1) * PUD_SIZE;
 	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
 		remain_entropy -= get_padding(&kaslr_regions[i]);
 
-- 
2.29.0.rc2.309.g374f81d7ae-goog