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Re: [oss-security] CVE-2021-20219 Linux kernel: improper synchronization in flush_to_ldisc() can lead to DoS

On 17.03.2021 18:17, Greg KH wrote:
On Wed, Mar 17, 2021 at 07:45:59PM +0530, Rohit Keshri wrote:
Hello Team,

A denial of service vulnerability was found in n_tty_receive_char_special
in drivers/tty/n_tty.c of the Linux kernel.  In this flaw a local attacker
with a normal user privilege could delay the loop (due to a changing
ldata->read_head, and a missing sanity check) and cause a threat to the
system availability.

'CVE-2021-20219' was assigned by Red Hat.

Acknowledgements: Evgenii Shatokhin (Virtuozzo Research LLC)

Really?  Not the tools or people that reported this issue and fixed it
in the community back in 2018?

The description is misleading, unfortunately.

RedHat backported that original fix (commit 3d63b7e4ae0d "n_tty: Fix stall at n_tty_receive_char_special().") long ago.

I just found that their backport was incomplete: one hunk of the patch was lost. This lead to the problem I reported: the reproducer program caused n_tty_receive_char() to loop forever. As a result, other processes could hang too.



greg k-h