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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Javier Martinez Canillas <javierm@redhat.com>
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Date: Tue, 12 May 2020 01:00:51 +0200
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Subject: [PATCH] envblk: Fix buffer overrun when attempting to shrink a
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variable value
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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If an existing variable is set with a value whose length is smaller than
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the current value, a memory corruption can happen due copying padding '#'
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characters outside of the environment block buffer.
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This is caused by a wrong calculation of the previous free space position
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after moving backward the characters that followed the old variable value.
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That position is calculated to fill the remaining of the buffer with the
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padding '#' characters. But since isn't calculated correctly, it can lead
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to copies outside of the buffer.
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The issue can be reproduced by creating a variable with a large value and
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then try to set a new value that is much smaller:
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$ grub2-editenv --version
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grub2-editenv (GRUB) 2.04
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$ grub2-editenv env create
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$ grub2-editenv env set a="$(for i in {1..500}; do var="b$var"; done; echo $var)"
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$ wc -c env
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1024 grubenv
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$ grub2-editenv env set a="$(for i in {1..50}; do var="b$var"; done; echo $var)"
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malloc(): corrupted top size
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Aborted (core dumped)
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$ wc -c env
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0 grubenv
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Resolves: rhbz#1836196
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Reported-by: Renaud Métrich <rmetrich@redhat.com>
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Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
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Patch-cc: Daniel Kiper <daniel.kiper@oracle.com>
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---
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grub-core/lib/envblk.c | 2 +-
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1 file changed, 1 insertion(+), 1 deletion(-)
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diff --git a/grub-core/lib/envblk.c b/grub-core/lib/envblk.c
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index 230e0e9d9ab..2e4e78b132d 100644
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--- a/grub-core/lib/envblk.c
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+++ b/grub-core/lib/envblk.c
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@@ -143,7 +143,7 @@ grub_envblk_set (grub_envblk_t envblk, const char *name, const char *value)
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/* Move the following characters backward, and fill the new
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space with harmless characters. */
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grub_memmove (p + vl, p + len, pend - (p + len));
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- grub_memset (space + len - vl, '#', len - vl);
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+ grub_memset (space - (len - vl), '#', len - vl);
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}
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else
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/* Move the following characters forward. */
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