fix crash - buffer overflow by one in fexpand #521
Merged
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
The symptom, on Windows, using LESSOPEN, was that "less" would sometimes (up to 5-10%) crash with exit code 116. gdb detected heap block overflow in free(filename) in lglob. The overflow is because fexpand_copy(s, NULL) return value (and allocated size) doesn't count the final \0, but fexpand_copy(s, x) does write a final \0. This appears to be a regression of commit 1626d06, where fexpand_copy returned the strlen of the result, and fexpand allocated n+1, but commit 1626d06 changed the allocation to n. Fix it by always counting the \0 - which appears to fix the crash. fexpand_copy is only used from fexpand (once to count, and once for the actual copy), so there's no risk elsewhere from this change.
|
Interestingly, it doesn't seem to crash when compiled with UCRT (either gcc or clang), but does crash when compiled with MSVCRT (either gcc or clang). Maybe the UCRT malloc leaves some space after the block, so it can avoid crashes with small overflows? Or maybe the allignment or selected block size is bigger with UCRT... EDIT: it also crashes with UCRT, just with different filename lengths. So I guess the UCRT malloc doesn't use exactly the same block sizes as MSVCRT. This PR fix the crash also with UCRT. |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
The symptom, on Windows, using LESSOPEN, was that "less" would sometimes (up to 5-10%) crash with exit code 116.
gdb detected heap block overflow in free(filename) in lglob.
The overflow is because fexpand_copy(s, NULL) return value (and allocated size) doesn't count the final \0, but fexpand_copy(s, x) does write a final \0.
This appears to be a regression of commit 1626d06, where fexpand_copy returned the strlen of the result, and fexpand allocated n+1, but commit 1626d06 changed the allocation to n.
Fix it by always counting the \0 - which appears to fix the crash.
fexpand_copy is only used from fexpand (once to count, and once for the actual copy), so there's no risk elsewhere from this change.