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Fri, 19 Jul 2024 08:50:07 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 0J1zK78ommafPAAAD6G6ig (envelope-from ); Fri, 19 Jul 2024 08:50:07 +0000 Message-ID: Date: Fri, 19 Jul 2024 10:50:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: warn potential return NULL for kmalloc_array and kvmalloc_array with __GFP_NOFAIL Content-Language: en-US To: Michal Hocko , Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, linux-mm@kvack.org, Barry Song , Uladzislau Rezki , Christoph Hellwig , Lorenzo Stoakes , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Roman Gushchin , Hyeonggon Yoo <42.hyeyoo@gmail.com> References: From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJkBREIBQkRadznAAoJECJPp+fMgqZkNxIQ ALZRqwdUGzqL2aeSavbum/VF/+td+nZfuH0xeWiO2w8mG0+nPd5j9ujYeHcUP1edE7uQrjOC Gs9sm8+W1xYnbClMJTsXiAV88D2btFUdU1mCXURAL9wWZ8Jsmz5ZH2V6AUszvNezsS/VIT87 AmTtj31TLDGwdxaZTSYLwAOOOtyqafOEq+gJB30RxTRE3h3G1zpO7OM9K6ysLdAlwAGYWgJJ V4JqGsQ/lyEtxxFpUCjb5Pztp7cQxhlkil0oBYHkudiG8j1U3DG8iC6rnB4yJaLphKx57NuQ PIY0Bccg+r9gIQ4XeSK2PQhdXdy3UWBr913ZQ9AI2usid3s5vabo4iBvpJNFLgUmxFnr73SJ KsRh/2OBsg1XXF/wRQGBO9vRuJUAbnaIVcmGOUogdBVS9Sun/Sy4GNA++KtFZK95U7J417/J Hub2xV6Ehc7UGW6fIvIQmzJ3zaTEfuriU1P8ayfddrAgZb25JnOW7L1zdYL8rXiezOyYZ8Fm ZyXjzWdO0RpxcUEp6GsJr11Bc4F3aae9OZtwtLL/jxc7y6pUugB00PodgnQ6CMcfR/HjXlae h2VS3zl9+tQWHu6s1R58t5BuMS2FNA58wU/IazImc/ZQA+slDBfhRDGYlExjg19UXWe/gMcl De3P1kxYPgZdGE2eZpRLIbt+rYnqQKy8UxlszsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZAUSmwUJDK5EZgAKCRAiT6fnzIKmZOJGEACOKABgo9wJXsbWhGWYO7mD 8R8mUyJHqbvaz+yTLnvRwfe/VwafFfDMx5GYVYzMY9TWpA8psFTKTUIIQmx2scYsRBUwm5VI EurRWKqENcDRjyo+ol59j0FViYysjQQeobXBDDE31t5SBg++veI6tXfpco/UiKEsDswL1WAr tEAZaruo7254TyH+gydURl2wJuzo/aZ7Y7PpqaODbYv727Dvm5eX64HCyyAH0s6sOCyGF5/p eIhrOn24oBf67KtdAN3H9JoFNUVTYJc1VJU3R1JtVdgwEdr+NEciEfYl0O19VpLE/PZxP4wX PWnhf5WjdoNI1Xec+RcJ5p/pSel0jnvBX8L2cmniYnmI883NhtGZsEWj++wyKiS4NranDFlA HdDM3b4lUth1pTtABKQ1YuTvehj7EfoWD3bv9kuGZGPrAeFNiHPdOT7DaXKeHpW9homgtBxj 8aX/UkSvEGJKUEbFL9cVa5tzyialGkSiZJNkWgeHe+jEcfRT6pJZOJidSCdzvJpbdJmm+eED w9XOLH1IIWh7RURU7G1iOfEfmImFeC3cbbS73LQEFGe1urxvIH5K/7vX+FkNcr9ujwWuPE9b 1C2o4i/yZPLXIVy387EjA6GZMqvQUFuSTs/GeBcv0NjIQi8867H3uLjz+mQy63fAitsDwLmR EP+ylKVEKb0Q2A== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 9344D40002 X-Stat-Signature: p9qgmwy48bdyt54t6y8ad53f1u3yy7uz X-Rspam-User: X-HE-Tag: 1721379009-340237 X-HE-Meta: U2FsdGVkX19h9AeKgnU6eMdrXrIkBrZwYO7a3NEzd8fPHg+GyOzoLA1hlSo+wweeOGmngF4GGklOtnXyxeXugd8MeB976Ug10FSAikKjmy6UZMx9sKBy6d04iliBt4CBThE8WvvjkLAhVtxqMPOJmrFBegAy1qt618sD+eKFFIIrROSZrWwA21lmsIPaIko4zsgcMAYrg9166H1lH7+hM28slbz3BKJyqdzVwFsS0eZyOV5gPAk2TEAbFpnHvlVj+6T1b0JG/e32MIHjqPKFiD+kECqRsbhC1MF5Tf9s53rspeKVwIjcjYUAKKfmSqP8tHIZfnMF79I1b/4oL8Qd/BBJdb6QUPEmVjXbatJkU8ToRIL7+guvA0b/04XUc2YYzsI8pV1EiGBGcD7m/RaMc1BMGV/erAHquQQxCEptzEoWmf/JsxhVHPXH2NV/RE8DS3jQfe5hBuK+j6+YluU2g529MDRyqYMKndR5sDIyylSKZxf/GvIQNFwpLlAmSEojJlPca9iGNiT8UjdGXZpzkM/tGBvry5EXqUI5uFIdUmoJ2CUV1E+X7oCgrMKxgCQyruInVrg4WWSHlHp+MNASuqVwnNjZiV3us0WuPhMG8eI6SPGgT61kyR3Ug1aiHnmH8i7WUXdDdq8Yun5+Mp407qB16aeXWIbhtle7IMrVsNEh0D4eOjlCrVKluM+/HIyW1hZKTm0dBlPaGxWFKwpkYQzxiflJ7Fzd9Tg4mR9q6XncWHOA94L6ISUuNeFe4PU74REJrV42dULDNyde7YQjfLh39MsYwLqu8gkyYl4F3uP6TR+0Bb9ekbzTLQefJtNYjItn+cykiO/tsE/POmimQcSjOi7SZ+os8qwZobxkGdt6pUA9q4h7JSUsT8IHkc7iczxCq5f/OEMo+YHLDn+vdMGUxoRmkS3lWwd7HVqZfX4o7h3BBewfPezl+Iz4pUq0XuCrECtFHXLQtSUjX8G P4jSMb0R vzTBGq5g6yDks7K64mVdwGqTuErYZBUlgXRI5Wb7Uk7kniOXE/SKTxLgcSqGEgdKaj/lbG8iSnyVeHzED9inMNpbtN+J9b/HEtHKpa9E6K0IK/uGQG/n75IJY5jaiR11pgQCWRUZEpy1jcu+crsNsUagrwdwTnli7gEfFtpsy+ixnRp9EhklEywxPZGYi6XOxfoujLOan6spijB4nt92o1FoSaRy9yu4JTPe45GKpVUyNPSUlyUFKmymRbgO2+mv4gB187UN5CO1F4m1Ca9fVxq6cFDlzCuWodTEJokAFU7Yq3P/zPbmPsa72+fqB4+kZ0XV9dtat2VZu9Qnd1C8JNp+rlAjqzkfPRfZT7NuQzPrntMb35WGtN+MHF0WFRp9aD1uMzUdTU/127Ked6TMvTrZAV81rH8wdkfhMOdljl+y0UQHdVxNe7tox5/dS5JVdD7/q2o0tdSuBYLb/HkJvRiBbfA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/19/24 10:01 AM, Michal Hocko wrote: > On Fri 19-07-24 19:51:06, Barry Song wrote: >> On Fri, Jul 19, 2024 at 7:42 PM Michal Hocko wrote: > [...] >> > It cannot reclaim itself and it cannot sleep to wait for the memory so >> > NOFAIL semantic is simply impossible. We have put a warning in place to >> >> this is still "right" behaviour to retry infinitely at least according >> to the doc of >> __GFP_NOFAIL. > > I do not agree that implementing busy loop in the kernel is the right > practice! I think there are multiple aspects to this. One is what's the right practice, another what's the most graceful recovery from a buggy code. We can all agree that __GFP_NOFAIL together with GFP_NOWAIT or other variants that can't sleep is wrong and unsupported. Hence we should put sufficient warnings in place to prevent such combinations from being used in the first place. But what if some callsite makes it into mainline without anyone seeing the warnings in the process and fixing them, because it's rare to hit? And then an attacker finds a way to hit it? We could fail the allocation which would probably crash afterwards, hopefully non-exploitably. We could BUG_ON() which would crash reliably, but still become a DoS vulnerability. Or we could busy loop, which might be also a DoS but maybe not so reliable because kswapd or other direct reclaiming processes could recover the situation (and the user could report the warning that was produced in the process). That wouldn't mean the busy loop is a correct and supported practice. It would just mean it's the least bad of the bad options we have to deal with an allocation that's wrong but we didn't catch soon enough in the development. Compared to the original problem at hand, kmalloc_array() of impossible size with __GFP_NOFAIL is not recoverable by busy looping so as I've already said I like most the idea of BUG_ON(). Yes it's a DoS vector if someone finds such a user triggerable allocation, but I don't see a better option. Same thing should probably happen with a __GFP_NOFAIL page allocation that requests >MAX_ORDER. >> I assume getting new memory by many retries is still >> possibly some other processes might be reclaiming or freeing memory >> then providing free memory to this one being stuck. > > No, I strongly disagree we should even pretend this is a supported > allocation strategy. NAK to any attempt to legalize it in some form. >