[PATCH] misc-utils: docfdisk.c: fix potential integer underflow in partition size calculation

[Anton Moryakov]

report of the static analyzer:
Possible integer underflow: right operand is tainted.
An integer underflow may occur due to arithmetic
operation (unsigned subtraction) between
variables 'totblocks' and 'block', where 'totblocks'
is in range { [0, 4294967295] }, and 'block' is tainted { [0, 4294967295] }

correct explained:
Added validation check before calculating remaining
space for partition. The issue occurred when setting
the last partition size to 0, which triggers calculation
'totblocks - block'. Without validation, if block >= totblocks,
this would result in integer underflow due to unsigned
arithmetic, potentially creating a partition with enormous
size and leading to device corruption.

Signed-off-by: Anton Moryakov <ant.v.moryakov at gmail.com>
---
 misc-utils/docfdisk.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/misc-utils/docfdisk.c b/misc-utils/docfdisk.c
index 486ce29..47e4ff9 100644
--- a/misc-utils/docfdisk.c
+++ b/misc-utils/docfdisk.c
@@ -253,8 +253,13 @@ int main(int argc, char **argv)
 	for (i = 0; i < npart; i++) {
 		ip = &(mh->Partitions[i]);
 		ip->firstUnit = cpu_to_le32(block);
-		if (!nblocks[i])
+		if (!nblocks[i]) {
+			if (block >= totblocks) {
+				printf("No space left on device for partition.\n");
+				return 1;
+			}
 			nblocks[i] = totblocks - block;
+		}
 		ip->virtualUnits = cpu_to_le32(nblocks[i]);
 		block += nblocks[i];
 		ip->lastUnit = cpu_to_le32(block-1);
-- 
2.39.2