Please describe the problem.

Not sure how I have missed this issue for a while, since now I have tried with almost year old 6.20170101-1+b1 -- the same story as with 6.20171003-g14ffdd779

I expect those files to be readable according to the 'shared' setting but definitely not writeable!

Please provide any additional information below.

$> sudo rm -rf tttt; mkdir tttt; ( cd tttt; git init --shared .; git annex init ; echo 123 >123; git annex add 123; git commit -m 'added 123'; ls -lL 123; ) 
Initialized empty shared Git repository in /tmp/tttt/.git/
init  ok
(recording state in git...)
add 123 ok
(recording state in git...)
[master (root-commit) b137ea6] added 123
 1 file changed, 1 insertion(+)
 create mode 120000 123
-rw-rw---- 1 yoh yoh 4 Oct 10 12:59 123

$> echo 11 >> tttt/123
# no error code

FWIW my umask is 077 but originally discovered on a system with umask 0002

I guess I need to annex fsck tons of repositories now :-/